Listview 颤振列表视图生成器

我正在尝试将RSS提要导入listView。我所面临的挑战是——RSS字段中的一些帖子有，而一些没有。我希望在ListTile中显示图像-我得到了一个错误，因为并非提要中的所有帖子都有-如果颤振没有找到标记，我如何显示默认图像-错误在这一行

前导：item.enclosure.url！=无效的thumbnailitem.enclosure.url:容器

这是我的代码：

import 'package:flutter/material.dart';
import 'package:webfeed/webfeed.dart';
import 'package:http/http.dart' as http;
import 'package:url_launcher/url_launcher.dart';
import 'package:cached_network_image/cached_network_image.dart';


class News extends StatefulWidget {
  News() : super ();
  final String title = 'Rss feed Demo';
  @override
  _NewsState createState() => _NewsState();
}

class _NewsState extends State<News> {
  //static const String FEED_URL = 'https://www.nasa.gov/rss/dyn/lg_image_of_the_day.rss';
  static const String FEED_URL = 'http://www.rssmix.com/u/11637744/rss.xml';
  RssFeed _feed;
  String _title;
  static const String loadingFeedMsg = 'Loading feed....';
  static const String feedLoadErrorMsg = 'Error loading feed';
  static const String feedOpenErrorMsg = 'Error opening Feed';
  static const String placeholderImg = 'images/no_image.png';
  GlobalKey<RefreshIndicatorState> _refreshKey;

  updateTitle(title){
    setState(() {
      _title = title;
    });
  }
  updateFeed(feed){
    setState(() {
      _feed = feed;
    });
  }

  Future<void> openFeed(String url) async {
    if(await canLaunch((url))){
      await launch(url, forceSafariVC: true, forceWebView: false,
      );
      return;

    }
    updateTitle(feedOpenErrorMsg);
  }

  load() async {
    updateTitle(loadingFeedMsg);
    loadFeed().then((result){
      if(null == result ||result.toString().isEmpty){
        updateTitle(feedLoadErrorMsg);
        return;

      }
      updateFeed(result);
      updateTitle(_feed.title);
    });
  }

  Future<RssFeed> loadFeed() async{
    try{
      final client = http.Client();
      final response = await client.get(FEED_URL);
      return RssFeed.parse(response.body);

    }catch (e){
      return null;

    }
  }

  @override
  void initState() {

    super.initState();
    _refreshKey = GlobalKey<RefreshIndicatorState>();
    updateTitle(widget.title);
    load();


  }
  title(title){
    return Text(
      title,
      style: TextStyle(fontSize: 15.0, fontWeight: FontWeight.w500),
      maxLines: 2,
      overflow: TextOverflow.ellipsis,
    );
  }

  subtitle(subtitle){
    return Text(
      subtitle,
      style: TextStyle(fontSize: 14.0, fontWeight: FontWeight.w200),
      maxLines: 1,
      overflow: TextOverflow.ellipsis,
    );
  }

  thumbnail(imageUrl){
    return Padding(
      padding: EdgeInsets.only(left: 15.0),
      child: CachedNetworkImage(
        placeholder: (context, url) => Image.asset(placeholderImg),
        imageUrl: imageUrl,
        height: 50,
        width: 70,
        alignment: Alignment.center,
        fit: BoxFit.fill,
      ),
    );
  }

  rightIcon(){
    return Icon(
      Icons.keyboard_arrow_right, color: Colors.grey, size: 30.0,
    );
  }

//  image(){
//    if (null = item.enclosure.url) {
//
//    }
//  }

  list(){
    return ListView.builder(
      itemCount: _feed.items.length,
      itemBuilder: (BuildContext context, int index){
        final item = _feed.items[index];
        //return ListTile(
        return ListTile(
          title: title(item.title),
          //subtitle: subtitle(item.description),
          subtitle: subtitle(item.description),

          leading: item.enclosure.url != null ? thumbnail(item.enclosure.url):Container(),
          //leading: thumbnail(item.enclosure.url),
          trailing: rightIcon(),
          contentPadding: EdgeInsets.all(5.0),
          onTap: () => openFeed(item.link),


        );

      },
    );
  }

  isFeedEmpty(){
    return null == _feed || null == _feed.items;
  }

  body(){
    return isFeedEmpty() ? Center(
      child: CircularProgressIndicator(),
    )
        :RefreshIndicator(
      key: _refreshKey,
      child: list(),
      onRefresh: () => load(),
    );



  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(_title),
      ),
      body:  body(),
    );
  }
}

---

将出现错误的行更改为此

leading: item.enclosure?.url != null
          ? thumbnail(item.enclosure.url)
          : SizedBox.shrink(), // Replace Container() with SizedBox.shrink()