我已经阅读了关于这个问题的链接。主要的解决方案是：

• 禁用默认操作栏。我做了...
• 把findViewById放在setContent后面。我做了...
• 将布局包含在主活动layout.xml文件中。

但仍然得到空指针回来。我越来越疯狂，因为我也看不到预览工具栏。

styles.xml

未设置操作栏。

<!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>
</resources>

my_toolbar.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/my_toolbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/colorPrimary"
    android:theme="@style/Theme.AppCompat.DayNight.DarkActionBar">
</android.support.v7.widget.Toolbar>

activity_maps.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <include
        android:id="@+id/my_toolbar"
        layout="@layout/my_toolbar"/>
    <!-- Add Google Maps Fragment to the MainActivity -->
    <fragment xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/map"
        android:name="com.google.android.gms.maps.SupportMapFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/my_toolbar"
        tools:context=".MapsActivity" />
        <!--android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"-->
    <Button
        android:id="@+id/btClear"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:text="@string/send_screenshot" />
</RelativeLayout>

MapsActivity.java

....

@Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_maps);
            // Get fused location client
            mFusedLocationClient = LocationServices.getFusedLocationProviderClient(this);
            Toolbar myToolbar = (Toolbar) findViewById(R.id.my_toolbar);
            myToolbar.setTitle("dio porco dio");
            setSupportActionBar(myToolbar);
            // Obtain the SupportMapFragment and get notified when the map is ready to be used.
            SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                    .findFragmentById(R.id.map);
            mapFragment.getMapAsync(this);
        }

....