Java setBackgroundColor异常

每当按下切换按钮时，我都试图更改活动的背景色，但我遇到了“由以下原因引起的错误：java.lang.NullPointerException:在空对象引用上尝试调用虚拟方法'void android.widget.LinearLayout.setBackgroundColor（int）'”错误

这是我的主要活动中的onCreate代码

delegate = AppCompatDelegate.create(this, this);
    delegate.onCreate(savedInstanceState);
    delegate.setContentView(R.layout.activity_main);
    Toolbar toolbar= (Toolbar) findViewById(R.id.toolbar);
    delegate.setSupportActionBar(toolbar); 

    mScreen = (LinearLayout) findViewById(R.id.main);
    mScreen.setBackgroundColor(getResources().getColor(R.color.red));

    final ToggleButton start = (ToggleButton) findViewById(R.id.button);
    start.setText("START");
    start.setOnCheckedChangeListener(new    CompoundButton.OnCheckedChangeListener() {
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
            if (isChecked) {
                startStepService();
                bindStepService();
                mScreen.setBackgroundColor(getResources().getColor(R.color.white));
                mIsRunning = true;
                start.setText("STOP");

            } else {
                unbindStepService();
                stopStepService();
                mScreen.setBackgroundColor(getResources().getColor(R.color.red));
                start.setText("START");
                mIsRunning = false;
            }
        }
    });

}

以及我的布局的XML。一个是协调器布局，其中包含我正在更改的布局

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="com.sp.service9.Pedometer">

    <android.support.design.widget.AppBarLayout
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/main"
        android:id="@+id/include" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="124dp"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical|center_horizontal"
        android:layout_margin="@dimen/fab_margin"
        android:onClick="showAlert"
        app:backgroundTint="?attr/colorPrimaryDark"
        app:layout_anchor="@+id/include"
        app:layout_anchorGravity="bottom|center_horizontal"
        app:srcCompat="@drawable/giftboxblue" />

</android.support.design.widget.CoordinatorLayout>

和main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main"
    android:paddingTop="50dp"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".Pedometer">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="60dp"
        app:srcCompat="@android:color/transparent"
        android:id="@+id/imageView2" />

    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <TextView
            android:text="Welcome, "
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:gravity="right"
            android:id="@+id/textView3"
            android:layout_weight="1" />

        <TextView
            android:text="TextView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/textView2"
            android:layout_weight="1" />

    </LinearLayout>

    <TextView
        android:textSize="30sp"
        android:layout_width="wrap_content"
        android:layout_height="59dp"
        android:layout_span="4"
        android:layout_gravity="center"
        android:id="@+id/StepTracker" />

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="268dp"
        android:layout_gravity="center">

        <ProgressBar
            android:id="@+id/circular_progress_bar"
            style="?android:attr/progressBarStyleHorizontal"
            android:layout_width="275dp"
            android:layout_height="260dp"
            android:indeterminate="false"
            android:progress="2"
            android:foregroundGravity="center"
            android:progressDrawable="@drawable/circular_progressbar"
            android:layout_weight="1"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true" />

        <TextView
            android:id="@+id/step_value"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_span="4"
            android:gravity="center_vertical|center_horizontal"
            android:text="0"
            android:textSize="60sp"
            android:layout_weight="1"
            android:layout_marginTop="88dp"
            android:layout_alignParentTop="true"
            android:layout_alignParentStart="true" />

    </RelativeLayout>

    <ToggleButton
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:text="Button" />


</LinearLayout>

---

您没有名为

R.id.main

的小部件

您有一个试图设置小部件ID的布局（

main.xml

）。但是，您的

标记会用不同的值（

@+ID/include

）覆盖该小部件ID

将Java代码更改为使用

R.id.include

，或者从

中删除

android:id

属性