title: Crash in Activity Transitions with SharedElement
body: <p>I'm using Activity transitions from a ViewPager (in the calling activity) with a shared element and content transitions as well. I'm getting this crash when re-entering to the calling activity: </p>
  
  <pre><code>java.lang.NullPointerException: Attempt to invoke virtual method 'void android.os.ResultReceiver.send(int, android.os.Bundle)' on a null object reference
    at android.app.EnterTransitionCoordinator.sendSharedElementDestination(EnterTransitionCoordinator.java:199)
    at android.app.EnterTransitionCoordinator.viewsReady(EnterTransitionCoordinator.java:123)
    at android.app.EnterTransitionCoordinator$2.onPreDraw(EnterTransitionCoordinator.java:148)
    at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:895)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2153)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1180)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6558)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:777)
    at android.view.Choreographer.doCallbacks(Choreographer.java:590)
    at android.view.Choreographer.doFrame(Choreographer.java:560)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:763)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:145)
    at android.app.ActivityThread.main(ActivityThread.java:5832)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
  </code></pre>
  
  <p>Also, Once going back, the screen begins to flicker continuously with a white screen flashing in and out.</p>
  
  <p>Here are my Transition flags:</p>
  
  <pre><code>&lt;item name="android:windowContentTransitions"&gt;true&lt;/item&gt;
  &lt;item name="android:windowActivityTransitions"&gt;true&lt;/item&gt;
  &lt;item name="android:windowAllowReturnTransitionOverlap"&gt;false&lt;/item&gt;
  </code></pre>
  
  <p>I tried setting Enter/Exit transitions both on the Calling and Called activity but no luck.</p>