Ignore events fired on missing React Native instances

This can happen if something gets unmounted before the event gets
dispatched. I'm not sure how this works out exactly but this
preserves previous behavior in this scenario.

src/renderers/native/ReactNative/ReactNativeEventEmitter.js

@@ -120,6 +120,11 @@ var ReactNativeEventEmitter = merge(ReactEventEmitterMixin, {
   ) {
     var nativeEvent = nativeEventParam || EMPTY_NATIVE_EVENT;
     var inst = ReactNativeComponentTree.getInstanceFromNode(rootNodeID);
+    if (!inst) {
+      // If the original instance is already gone, we don't have to dispatch
+      // any events.
+      return;
+    }
     ReactUpdates.batchedUpdates(function() {
       ReactNativeEventEmitter.handleTopLevel(
         topLevelType,