Check if child view != null before dropping (facebook#20465)

Summary:
Fixes our top crash when framework try drop a view from parent, but it's a null (already removed etc.).

Fixes facebook#20288
Pull Request resolved: facebook#20465

Differential Revision: D10113976

Pulled By: hramos

fbshipit-source-id: 34f5654f3bdbc63eb7f7d0b5c94885576fc3cdcd

Author: Artur Chrusciel

ReactAndroid/src/main/java/com/facebook/react/uimanager/NativeViewHierarchyManager.java

@@ -544,7 +544,9 @@ protected synchronized void dropView(View view) {
       ViewGroupManager viewGroupManager = (ViewGroupManager) viewManager;
       for (int i = viewGroupManager.getChildCount(viewGroup) - 1; i >= 0; i--) {
         View child = viewGroupManager.getChildAt(viewGroup, i);
-        if (mTagsToViews.get(child.getId()) != null) {
+        if (child == null) {
+            FLog.e(TAG, "Unable to drop null child view");
+        } else if (mTagsToViews.get(child.getId()) != null) {
           dropView(child);
         }
       }