Android androidx.appcompat.widget.SwitchCompat 崩溃

java.lang.NullPointerException: Attempt to invoke interface method 'int java.lang.CharSequence.length()' on a null object reference
  at android.text.StaticLayout.<init>(StaticLayout.java:422)
  at android.support.v7.widget.SwitchCompat.makeLayout(SwitchCompat.java:602)
  at android.support.v7.widget.SwitchCompat.onMeasure(SwitchCompat.java:522)
  at android.view.View.measure(View.java:19923)
  at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1489)
  at android.widget.LinearLayout.measureVertical(LinearLayout.java:775)
  at android.widget.LinearLayout.onMeasure(LinearLayout.java:657)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1489)
  at android.widget.LinearLayout.measureVertical(LinearLayout.java:775)
  at android.widget.LinearLayout.onMeasure(LinearLayout.java:657)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1489)
  at android.widget.LinearLayout.measureVertical(LinearLayout.java:775)
  at android.widget.LinearLayout.onMeasure(LinearLayout.java:657)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1489)
  at android.widget.LinearLayout.measureVertical(LinearLayout.java:775)
  at android.widget.LinearLayout.onMeasure(LinearLayout.java:657)
  at android.view.View.measure(View.java:19923)
  at android.widget.ScrollView.measureChildWithMargins(ScrollView.java:1330)
  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
  at android.widget.ScrollView.onMeasure(ScrollView.java:352)
  at android.view.View.measure(View.java:19923)
  at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1489)
  at android.widget.LinearLayout.measureVertical(LinearLayout.java:775)
  at android.widget.LinearLayout.onMeasure(LinearLayout.java:657)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6139)
  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
  at com.android.internal.policy.DecorView.onMeasure(DecorView.java:729)
  at android.view.View.measure(View.java:19923)
  at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2436)
  at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1482)
  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1741)
  at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1366)
  at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6773)
  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:926)
  at android.view.Choreographer.doCallbacks(Choreographer.java:735)
  at android.view.Choreographer.doFrame(Choreographer.java:667)
  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:912)
  at android.os.Handler.handleCallback(Handler.java:761)
  at android.os.Handler.dispatchMessage(Handler.java:98)
  at android.os.Looper.loop(Looper.java:199)
  at android.app.ActivityThread.main(ActivityThread.java:6585)
  at java.lang.reflect.Method.invoke(Native Method)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:986)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:876)

真是莫名其妙，这到底是为何呢？

 原来这个老项目的Activity 的主题引用竟然是：

android:theme="@android:style/Theme.DeviceDefault.DialogWhenLarge"

但是这个SwitchCompat 是V7 包才有的，所以引用的主题必须是Theme.AppCompat 的子类才行，修改如下：

android:theme="@style/AppTheme"

这样就好了