Why am i getting this error ->Unhandled Exception: type 'Null' is not a subtype of type 'String' in type cast on running the firebase_in_app_messaging code?

[harshkumarkhatri]

Column(
      children: [
        ElevatedButton(
            onPressed: () {
              print("before event triggered");
              FirebaseInAppMessaging.instance.triggerEvent("");
              print("event triggered");

              FirebaseMessaging.instance.sendMessage();
              print("message sent");
              FirebaseMessaging.instance.getInitialMessage();
              print("got initial message");
            },
            child: Text("Press for notification")),
        const Divider(height: 1),
        Expanded(
          child: Stack(
            children: [
              _PostList(),
              _AnimatedCommentBox(),
            ],
          ),
        ),
      ],
    )

Stacktrace

Error is coming from this file

I have used this article to implement the functionality.
https://medium.com/flutterdevs/explore-firebase-in-app-messaging-in-flutter-eaf219d208b1