Inside of setupCallHandler() changed it to return from await.

lib/src/platform/src/background_android_isolate_platform.dart

@@ -20,17 +20,14 @@ class BackgroundIsolateAndroidPlatform {
     _methodChannel.setMethodCallHandler((call) async {
       switch (call.method) {
         case PlatformMethod.pushOnBackgroundMessage:
-          await _onPushBackgroundMessage(call.arguments as RemoteMessage);
-          return;
+          return _onPushBackgroundMessage(call.arguments as RemoteMessage);
         default:
-          throw PlatformException(
-              code: 'invalid_method', message: 'No such method ${call.method}');
+          throw PlatformException(code: 'invalid_method', message: 'No such method ${call.method}');
       }
     });
   }
 
-  static final BackgroundIsolateAndroidPlatform _platform =
-      BackgroundIsolateAndroidPlatform._internal();
+  static final BackgroundIsolateAndroidPlatform _platform = BackgroundIsolateAndroidPlatform._internal();
 
   /// @nodoc
   /// Singleton instance of BackgroundIsolateAndroidPlatform
@@ -40,19 +37,15 @@ class BackgroundIsolateAndroidPlatform {
   /// A method channel used to communicate with the user's app isolate
   /// we explicitly launched when a RemoteMessage is received.
   /// Used only on Android.
-  final MethodChannel _methodChannel = MethodChannel(
-      'io.ably.flutter.plugin.background', StandardMethodCodec(Codec()));
+  final MethodChannel _methodChannel = MethodChannel('io.ably.flutter.plugin.background', StandardMethodCodec(Codec()));
 
-  final PushNotificationEventsInternal _pushNotificationEvents =
-      Push.notificationEvents as PushNotificationEventsInternal;
+  final PushNotificationEventsInternal _pushNotificationEvents = Push.notificationEvents as PushNotificationEventsInternal;
 
-  Future<void> _onPushBackgroundMessage(RemoteMessage remoteMessage) async =>
-      _pushNotificationEvents.handleBackgroundMessage(remoteMessage);
+  Future<void> _onPushBackgroundMessage(RemoteMessage remoteMessage) async => _pushNotificationEvents.handleBackgroundMessage(remoteMessage);
 
   /// @nodoc
   /// Equivalent of [Platform.invokePlatformMethod] which cannot be used here
   /// because we may not be able to acquire [Platform] instance here, so we
   /// need to use a raw [MethodChannel] communication
-  Future<T?> invokeMethod<T>(String method, [dynamic arguments]) async =>
-      _methodChannel.invokeMethod<T>(method, arguments);
+  Future<T?> invokeMethod<T>(String method, [dynamic arguments]) async => _methodChannel.invokeMethod<T>(method, arguments);
 }