Android studio 断言失败：布尔表达式不能为null？？如何解决这个问题？

当我尝试运行此应用程序时，出现以下错误

Failed assertion: boolean expression must not be null
    The relevant error-causing widget was: 
      MyApp file:///E:/Flutter/quizmakerapp/lib/main.dart:9:10
    When the exception was thrown, this was the stack: 
    #0      _MyAppState.build (package:quizapp2/main.dart:44:13)
    #1      StatefulElement.build (package:flutter/src/widgets/framework.dart:4663:28)
    #2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4546:15)
    #3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
    #4      Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)

嗨，我正在用一个Flitter和android studio制作一个测验应用程序。当我实现这个应用程序时，我遇到了这样一个错误。这些是我的文件，当我试图运行此程序时，会出现上述错误。我尝试了一些解决方案，但对我来说不起作用。我也不太会飞。对这个问题有什么想法吗

main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:quizapp2/helper/authenticate.dart';
import 'package:quizapp2/helper/constants.dart';
import 'package:quizapp2/views/home.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  bool isUserLoggedIn = false;

  @override
  void initState() {
    getLoggedInState();
    super.initState();
  }

  getLoggedInState() async {
    await Constants.getUerLoggedInSharedPreference().then((value) {
      setState(() {
        isUserLoggedIn = value;
      });
    });
  }

  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      title: 'Quiz App',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: isUserLoggedIn ? Home() : Authenticate(),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:quizapp2/services/database.dart';
import 'package:quizapp2/views/create_quiz.dart';
import 'package:quizapp2/views/quiz_play.dart';
import 'package:quizapp2/widget/widget.dart';

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  Stream quizStream;
  DatabaseService databaseService = new DatabaseService();

  Widget quizList() {
    return Container(
      child: Column(
        children: [
          StreamBuilder(
            stream: quizStream,
            builder: (context, snapshot) {
              return snapshot.data == null
                  ? Container()
                  : ListView.builder(
                      shrinkWrap: true,
                      physics: ClampingScrollPhysics(),
                      itemCount: snapshot.data.documents.length,
                      itemBuilder: (context, index) {
                        return QuizTile(
                          noOfQuestions: snapshot.data.documents.length,
                          imageUrl:
                              snapshot.data.documents[index].data['quizImgUrl'],
                          title:
                              snapshot.data.documents[index].data['quizTitle'],
                          description:
                              snapshot.data.documents[index].data['quizDesc'],
                          id: snapshot.data.documents[index].data["id"],
                        );
                      });
            },
          )
        ],
      ),
    );
  }

  @override
  void initState() {
    databaseService.getQuizData().then((value) {
      quizStream = value;
      setState(() {});
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: AppLogo(),
        brightness: Brightness.light,
        elevation: 0.0,
        backgroundColor: Colors.transparent,
        //brightness: Brightness.li,
      ),
      body: quizList(),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          Navigator.push(
              context, MaterialPageRoute(builder: (context) => CreateQuiz()));
        },
      ),
    );
  }
}

class QuizTile extends StatelessWidget {
  final String imageUrl, title, id, description;
  final int noOfQuestions;

  QuizTile(
      {@required this.title,
      @required this.imageUrl,
      @required this.description,
      @required this.id,
      @required this.noOfQuestions});

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: (){
        Navigator.push(context, MaterialPageRoute(
          builder: (context) => QuizPlay(id)
        ));
      },
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24),
        height: 150,
        child: ClipRRect(
          borderRadius: BorderRadius.circular(8),
          child: Stack(
            children: [
              Image.network(
                imageUrl,
                fit: BoxFit.cover,
                width: MediaQuery.of(context).size.width,
              ),
              Container(
                color: Colors.black26,
                child: Center(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                      Text(
                        title,
                        style: TextStyle(
                            fontSize: 18,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      ),
                      SizedBox(height: 4,),
                      Text(
                        description,
                        style: TextStyle(
                            fontSize: 13,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      )
                    ],
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:quizapp2/views/signin.dart';
import 'package:quizapp2/views/signup.dart';

class Authenticate extends StatefulWidget {
  @override
  _AuthenticateState createState() => _AuthenticateState();
}

class _AuthenticateState extends State<Authenticate> {
  bool showSignIn = true;

  void toggleView() {
    setState(() {
      showSignIn = !showSignIn;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (showSignIn) {
      return SignIn(toogleView: toggleView);
    } else {
      return SignUp(toogleView: toggleView);
    }
  }
}

导入“包装：颤振/材料.省道”；
导入“包：flifter/services.dart”；
导入“package:quizapp2/helper/authenticate.dart”；
导入“package:quizapp2/helper/constants.dart”；
导入“package:quizapp2/views/home.dart”；
导入“package:shared_preferences/shared_preferences.dart”；
void main（）{
runApp（MyApp（））；
}
类MyApp扩展了StatefulWidget{
//此小部件是应用程序的根。
@凌驾
_MyAppState createState（）=>\u MyAppState（）；
}
类MyAppState扩展了状态{
bool isUserLoggedIn=false；
@凌驾
void initState（）{
getLoggedInState（）；
super.initState（）；
}
getLoggedInState（）异步{
wait Constants.getuerloggedinSharedReference（）.then（（值）{
设置状态（）{
isUserLoggedIn=值；
});
});
}
@凌驾
小部件构建（构建上下文）{
返回材料PP(
标题：“问答应用程序”，
debugShowCheckedModeBanner:false，
主题：主题数据(
视觉密度：视觉密度。自适应平台密度，
),
home:isUserLoggedIn？home（）：Authenticate（），
);
}
}

本垒打。飞镖

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:quizapp2/helper/authenticate.dart';
import 'package:quizapp2/helper/constants.dart';
import 'package:quizapp2/views/home.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  bool isUserLoggedIn = false;

  @override
  void initState() {
    getLoggedInState();
    super.initState();
  }

  getLoggedInState() async {
    await Constants.getUerLoggedInSharedPreference().then((value) {
      setState(() {
        isUserLoggedIn = value;
      });
    });
  }

  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      title: 'Quiz App',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: isUserLoggedIn ? Home() : Authenticate(),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:quizapp2/services/database.dart';
import 'package:quizapp2/views/create_quiz.dart';
import 'package:quizapp2/views/quiz_play.dart';
import 'package:quizapp2/widget/widget.dart';

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  Stream quizStream;
  DatabaseService databaseService = new DatabaseService();

  Widget quizList() {
    return Container(
      child: Column(
        children: [
          StreamBuilder(
            stream: quizStream,
            builder: (context, snapshot) {
              return snapshot.data == null
                  ? Container()
                  : ListView.builder(
                      shrinkWrap: true,
                      physics: ClampingScrollPhysics(),
                      itemCount: snapshot.data.documents.length,
                      itemBuilder: (context, index) {
                        return QuizTile(
                          noOfQuestions: snapshot.data.documents.length,
                          imageUrl:
                              snapshot.data.documents[index].data['quizImgUrl'],
                          title:
                              snapshot.data.documents[index].data['quizTitle'],
                          description:
                              snapshot.data.documents[index].data['quizDesc'],
                          id: snapshot.data.documents[index].data["id"],
                        );
                      });
            },
          )
        ],
      ),
    );
  }

  @override
  void initState() {
    databaseService.getQuizData().then((value) {
      quizStream = value;
      setState(() {});
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: AppLogo(),
        brightness: Brightness.light,
        elevation: 0.0,
        backgroundColor: Colors.transparent,
        //brightness: Brightness.li,
      ),
      body: quizList(),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          Navigator.push(
              context, MaterialPageRoute(builder: (context) => CreateQuiz()));
        },
      ),
    );
  }
}

class QuizTile extends StatelessWidget {
  final String imageUrl, title, id, description;
  final int noOfQuestions;

  QuizTile(
      {@required this.title,
      @required this.imageUrl,
      @required this.description,
      @required this.id,
      @required this.noOfQuestions});

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: (){
        Navigator.push(context, MaterialPageRoute(
          builder: (context) => QuizPlay(id)
        ));
      },
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24),
        height: 150,
        child: ClipRRect(
          borderRadius: BorderRadius.circular(8),
          child: Stack(
            children: [
              Image.network(
                imageUrl,
                fit: BoxFit.cover,
                width: MediaQuery.of(context).size.width,
              ),
              Container(
                color: Colors.black26,
                child: Center(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                      Text(
                        title,
                        style: TextStyle(
                            fontSize: 18,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      ),
                      SizedBox(height: 4,),
                      Text(
                        description,
                        style: TextStyle(
                            fontSize: 13,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      )
                    ],
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:quizapp2/views/signin.dart';
import 'package:quizapp2/views/signup.dart';

class Authenticate extends StatefulWidget {
  @override
  _AuthenticateState createState() => _AuthenticateState();
}

class _AuthenticateState extends State<Authenticate> {
  bool showSignIn = true;

  void toggleView() {
    setState(() {
      showSignIn = !showSignIn;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (showSignIn) {
      return SignIn(toogleView: toggleView);
    } else {
      return SignUp(toogleView: toggleView);
    }
  }
}

导入“包装：颤振/材料.省道”；
导入“package:quizapp2/services/database.dart”；
导入“package:quizapp2/views/create_quick.dart”；
导入“包：quizapp2/views/quick_play.dart”；
导入“package:quizapp2/widget/widget.dart”；
类Home扩展了StatefulWidget{
@凌驾
_HomeState createState（）=>\u HomeState（）；
}
类(HomeState扩展状态){
溪流；
DatabaseService DatabaseService=新的DatabaseService（）；
Widget quizList（）{
返回容器(
子：列(
儿童：[
StreamBuilder(
溪流：quizStream，
生成器：（上下文，快照）{
返回snapshot.data==null
？容器（）
：ListView.builder(
收缩膜：对，
物理：ClampingScrollPhysics（），
itemCount:snapshot.data.documents.length，
itemBuilder：（上下文，索引）{
回程快件(
noOfQuestions:snapshot.data.documents.length，
图像URL：
snapshot.data.documents[index].data['quizImgUrl']，
标题：
snapshot.data.documents[index].data['quizTitle']，
说明：
snapshot.data.documents[index].data['quizDesc']，
id:snapshot.data.documents[index].data[“id”]，
);
});
},
)
],
),
);
}
@凌驾
void initState（）{
databaseService.getQuizData（）.then（（值）{
quizStream=值；
setState（（）{}）；
});
super.initState（）；
}
@凌驾
小部件构建（构建上下文）{
返回脚手架(
背景颜色：Colors.white，
appBar:appBar(
标题：AppLogo（），
亮度：亮度，亮度，
标高：0.0，
背景颜色：颜色。透明，
//亮度：亮度。李，
),
正文：quizList（），
浮动操作按钮：浮动操作按钮(
子：图标（Icons.add），
已按下：（）{
导航器。推(
context，materialpage（builder:（context）=>createquick（））；
},
),
);
}
}
类QuizTile扩展了无状态小部件{
最终字符串imageUrl、标题、id、说明；
期末考试；
奎茨特尔(
{@需要这个标题，
@需要此.imageUrl，
@需要此说明，
@需要这个.id，
@需要此选项。noOfQuestions}）；
@凌驾
小部件构建（构建上下文）{
返回手势检测器(
onTap:（）{
Navigator.push（上下文、MaterialPage路由）(
生成器：（上下文）=>QuizPlay（id）
));
},
子：容器(
填充：边缘组。对称（水平：24），
身高：150，
孩子：ClipRRect(
边界半径：边界半径。圆形（8），
子：堆栈(
儿童：[
图像网络(
imageUrl，
适合：BoxFit.cover，
宽度：MediaQuery.of（context）.size.width，
),
容器(
颜色：颜色。黑色，
儿童：中心(
子：列(
mainAxisAlignment:mainAxisAlignment.center，
儿童：[
正文(
标题
样式：TextStyle(
尺码：18，
颜色：颜色，白色，
fontWeight:fontWeight.w500），
),
尺寸箱（高度：4，），
正文(
描述
样式：TextStyle(
尺寸：13，
颜色：颜色，白色，
fontWeight:fontWeight.w500），
)
],
),
),
)
],
),
),
),
);
}
}

验证。dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:quizapp2/helper/authenticate.dart';
import 'package:quizapp2/helper/constants.dart';
import 'package:quizapp2/views/home.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  bool isUserLoggedIn = false;

  @override
  void initState() {
    getLoggedInState();
    super.initState();
  }

  getLoggedInState() async {
    await Constants.getUerLoggedInSharedPreference().then((value) {
      setState(() {
        isUserLoggedIn = value;
      });
    });
  }

  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      title: 'Quiz App',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: isUserLoggedIn ? Home() : Authenticate(),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:quizapp2/services/database.dart';
import 'package:quizapp2/views/create_quiz.dart';
import 'package:quizapp2/views/quiz_play.dart';
import 'package:quizapp2/widget/widget.dart';

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  Stream quizStream;
  DatabaseService databaseService = new DatabaseService();

  Widget quizList() {
    return Container(
      child: Column(
        children: [
          StreamBuilder(
            stream: quizStream,
            builder: (context, snapshot) {
              return snapshot.data == null
                  ? Container()
                  : ListView.builder(
                      shrinkWrap: true,
                      physics: ClampingScrollPhysics(),
                      itemCount: snapshot.data.documents.length,
                      itemBuilder: (context, index) {
                        return QuizTile(
                          noOfQuestions: snapshot.data.documents.length,
                          imageUrl:
                              snapshot.data.documents[index].data['quizImgUrl'],
                          title:
                              snapshot.data.documents[index].data['quizTitle'],
                          description:
                              snapshot.data.documents[index].data['quizDesc'],
                          id: snapshot.data.documents[index].data["id"],
                        );
                      });
            },
          )
        ],
      ),
    );
  }

  @override
  void initState() {
    databaseService.getQuizData().then((value) {
      quizStream = value;
      setState(() {});
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: AppLogo(),
        brightness: Brightness.light,
        elevation: 0.0,
        backgroundColor: Colors.transparent,
        //brightness: Brightness.li,
      ),
      body: quizList(),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          Navigator.push(
              context, MaterialPageRoute(builder: (context) => CreateQuiz()));
        },
      ),
    );
  }
}

class QuizTile extends StatelessWidget {
  final String imageUrl, title, id, description;
  final int noOfQuestions;

  QuizTile(
      {@required this.title,
      @required this.imageUrl,
      @required this.description,
      @required this.id,
      @required this.noOfQuestions});

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: (){
        Navigator.push(context, MaterialPageRoute(
          builder: (context) => QuizPlay(id)
        ));
      },
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24),
        height: 150,
        child: ClipRRect(
          borderRadius: BorderRadius.circular(8),
          child: Stack(
            children: [
              Image.network(
                imageUrl,
                fit: BoxFit.cover,
                width: MediaQuery.of(context).size.width,
              ),
              Container(
                color: Colors.black26,
                child: Center(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                      Text(
                        title,
                        style: TextStyle(
                            fontSize: 18,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      ),
                      SizedBox(height: 4,),
                      Text(
                        description,
                        style: TextStyle(
                            fontSize: 13,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      )
                    ],
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:quizapp2/views/signin.dart';
import 'package:quizapp2/views/signup.dart';

class Authenticate extends StatefulWidget {
  @override
  _AuthenticateState createState() => _AuthenticateState();
}

class _AuthenticateState extends State<Authenticate> {
  bool showSignIn = true;

  void toggleView() {
    setState(() {
      showSignIn = !showSignIn;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (showSignIn) {
      return SignIn(toogleView: toggleView);
    } else {
      return SignUp(toogleView: toggleView);
    }
  }
}

导入“包装：颤振/材料.省道”；
导入“包：quizapp2/views/signin.dart”；
导入“package:quizapp2/views/signup.dart”；