Fix crash when releasing RN views

Summary: There are cases where we're trying to drop a view that is not associated with a ViewManager. This is likely caused by race conditions that can occur if we're dropping a view from JS (when it's no longer used) but at the same time dropping it from native (when layout animation ends, when the rootview gets unmounted). In either of those cases, it should be safe to ignore the drop operation because the view was likely dropped already.

Reviewed By: mdvacca

Differential Revision: D13036643

fbshipit-source-id: 260ffb56d32a0d670ad08f449b8df165b2533195

Author: ayc1

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

@@ -562,6 +562,11 @@ protected synchronized final void addRootViewGroup(
    */
   protected synchronized void dropView(View view) {
     UiThreadUtil.assertOnUiThread();
+    if (mTagsToViewManagers.get(view.getId()) == null) {
+      // This view has already been dropped (likely due to a threading issue caused by async js
+      // execution). Ignore this drop operation.
+      return;
+    }
     if (!mRootTags.get(view.getId())) {
       // For non-root views we notify viewmanager with {@link ViewManager#onDropInstance}
       resolveViewManager(view.getId()).onDropViewInstance(view);