Flutter firebase onBackgroundMessage throws exception

[bilalgodesto]

When i add this function to my code.

FirebaseMessaging.onBackgroundMessage(
(message) => _handleBG(message, currentUserId));
I get exception

[ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'toRawHandle' was called on null

onMessage method below is working fine.

FirebaseMessaging.onMessage.listen((event) async {
// event.messageType
print('hearing to foreground messsage');
if (event.data['messageType'] == 'chat') {
await ChatHelper.handleMessageReceived(event, context, currentUserId);
//setState(() {});

      }
    });

    FirebaseMessaging.onBackgroundMessage(
        (message) => _handleBG(message, currentUserId));