Potential error with sending notifications

Hi there, great package, it's proving very useful so far. I'm having issues trying to send a notification without error and have tried most of the API surface. I could do with understanding whether this is a bug in the implementation or if I'm doing something wrong.

Approach 1:

var messaging = initializeApp().messaging();

var result = await messaging
        .send(TokenMessage(
          token: token,
          android: AndroidConfig(
            notification: AndroidNotification(title: "title", body: "body"),
          ),
        ))
        .catchError((e) => Exception(e));

This returns the error: NoSuchMethodError: method not found: 'toString' on null. I know the app and token are non-null and that this is the point of error. Here I'm using the AndroidConfig as a potential workaround to using notification: NotificationPayload as I was having to extend NotificationPayload for my code to be valid and was trying to rule everything out.

Approach 2 (based on NodeJS documentation):

    var message = {
      'Message': {
        'token': token,
        'notification': {'title': 'title', 'body': 'body'}
      }
    };

    var method1 = await messaging.send(message).catchError((e) => print(e));

This returns the error: Error: Exactly one of topic, token or condition is required which I assume is from Firebase itself.

I have also tried permutations with .sendToDevice(), .sendToDeviceGroup() and others without luck, normally receiving the NoSuchMethodError: method not found: 'toString' on null error.

Is there a working example I could test?