Add null check for gesture ended notifier

Summary:
The `notifyNativeGestureEnded` API is added to notify user gesture ended, so that any optimization we had during handling the gesture can be restored.

It's possible that when the gesture finishes, the RootView is already unmounted from the native side. This might happen when user starts a gesture that caused leave of the RN screen, or close the app.

Changelog:
[Android][Internal] - Avoid NPE for gesture notifier

Reviewed By: javache

Differential Revision: D35902523

fbshipit-source-id: 9bb5819a53dd053290031eebaae1b8f0318ae534

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/NativeGestureUtil.java

@@ -9,6 +9,7 @@
 
 import android.view.MotionEvent;
 import android.view.View;
+import com.facebook.react.uimanager.RootView;
 import com.facebook.react.uimanager.RootViewUtil;
 
 /** Utilities for native Views that interpret native gestures (e.g. ScrollView, ViewPager, etc.). */
@@ -35,6 +36,9 @@ public static void notifyNativeGestureStarted(View view, MotionEvent event) {
  * @param event the MotionEvent that caused the gesture to be ended
  */
  public static void notifyNativeGestureEnded(View view, MotionEvent event) {
- RootViewUtil.getRootView(view).onChildEndedNativeGesture(view, event);
+ RootView rootView = RootViewUtil.getRootView(view);
+ if (rootView != null) {
+ rootView.onChildEndedNativeGesture(view, event);
+ }
  }
 }