I made the call to the onPushBackgroundMessage await for the handler to complete.

I changed handleBackgroundMessage() to become a Future<void> and async/await.

I made the BackgroundMessageHandler a Future<void> instead of void so that we can async/await for it to be completed.

handleBackgroundMessage now handles when _onBackgroundMessage is defined as void or Future<void>  It checks to see if it's a Future and then awaits it if it is.

Changed the cast on BackgroundMessageHandler to dynamic, this allows it to be defined as both void and Future<void>