Emit warning when dispatching event to null target

Reviewed By: ShikaSD

Differential Revision: D33767454

fbshipit-source-id: c2f10b6b857544c06780fb13b66417d12f9b8d2c

ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

@@ -124,6 +124,10 @@ void UIManagerBinding::dispatchEvent(
     }()
     : jsi::Value::null();
 
+  if (instanceHandle.isNull()) {
+    LOG(WARNING) << "instanceHandle is null, event will be dropped";
+  }
+
   auto &eventHandlerWrapper =
       static_cast<EventHandlerWrapper const &>(*eventHandler_);