When handling multiple messages in a row, UnifiedPushReceiver could create multiple FlutterEngines: onReceive() is called, but Plugin.pluginChannel stays null before Plugin.onAttachedToEngine() is invoked.
Ensure we create only a single FlutterEngine per UnifiedPushReceiver.
When handling multiple messages in a row, UnifiedPushReceiver could create multiple FlutterEngines: onReceive() is called, but Plugin.pluginChannel stays null before Plugin.onAttachedToEngine() is invoked.
Ensure we create only a single FlutterEngine per UnifiedPushReceiver.