The argument type 'Null Function(DataSnapshot)' can't be assigned to the type 'FutureOr<dynamic> Function(DatabaseEvent)' in v 4.6.1

I needed to check if the firebase database had the user,

if (firebaseUser != null) {
      usersRef
          .child(firebaseUser.uid)
          .once()
          .then((DataSnapshot snap){
                if (snap.value != null) 
                {
                  Navigator.pushNamedAndRemoveUntil(
                      context, HomeScreen.idScreen, (route) => false);
                  displayToastMessage("You are logged-in now", context);
                } else {
                  _firebaseAuth.signOut();
                  displayToastMessage(
                      "No records found. Create a new account", context);
                }
                });
}

I added as FutureOr Function(DatabaseEvent)); yet I

I got unhandled exception when I run the code. I would be glad if someone has also encountered a similar error and could help.

I think the error is due to the fact Firebase Database API got an update in the 'onValue' event handler. the 'once()' method is now providing a 'DatabaseEvent' instead of a 'DataSnapshot'.

You could try this instead:

if (firebaseUser != null) {
usersRef
    .child(firebaseUser.uid)
    .get()
    .then((DatabaseEvent event) {
        DataSnapshot snap = event.snapshot;
        if (snap.value != null) {
            Navigator.pushNamedAndRemoveUntil(context, HomeScreen.idScreen, (route) => false);
            displayToastMessage("You are logged-in now", context);
        } else {
            _firebaseAuth.signOut();
            displayToastMessage("No records found. Create a new account", context);
        }
    });

}

In this revised snippet, 'DatabaseEvent event' is provided to the callback function for the 'then' method. The 'DataSnapshot snap is' then extracted from the 'event.snapshot'. The rest of the code remains the same.

This solution assumes you are using version 4.6.1 of the Firebase Database API as per your question.