main.dart

    FirebaseMessaging.onMessage.listen((RemoteMessage event) {
      print("message recieved");
      print(event.notification!.body);
      showDialog(
          context: context,
          builder: (BuildContext context) {
            return AlertDialog(
              title: Text("Notification"),
              content: Text(event.notification!.body!),
              actions: [
                TextButton(
                  child: Text("Ok"),
                  onPressed: () {
                    Navigator.of(context).pop();
                  },
                )
              ],
            );
          });
    });