return a bool value from a Future<bool> function in flutter

My Question is a bit similar to How can I get my bool value out of a future<bool>? but i couldn’t get an aswer to my question from it. I have the function in a seperate file because i also want to use it elsewhere in my code. I am trying to create a function that checks if the current user is the creator of the document. For that i use an async method in my function so it has to be Future but if I run my App i get this error:

The field userId in my Database is defined and saves the userId of the Author when it is created.

this is the function:

 Future<bool> isUserAuthor(documentId) async {
      var userId = '';
      String? currentUserId = FirebaseAuth.instance.currentUser?.uid;
      var collection = FirebaseFirestore.instance.collection('notes');
      var docSnapshot = await collection.doc(documentId).get();
      if (docSnapshot.exists) {
        userId = docSnapshot.data()?['userId'];
      } else {
        userId = 'no User available';
      }
      if (userId == currentUserId) {
        return true;
      } else {
        return false;
      }
    }

and this is where i use it:

typedef NoteCallback = void Function(CloudNote note);

class NotesListView extends StatelessWidget {
  final Iterable<CloudNote> notes;
  final NoteCallback onDeleteNote;
  final NoteCallback onTap;


  const NotesListView({
    Key? key,
    required this.notes,
    required this.onDeleteNote,
    required this.onTap,
  }) : super(key: key);
  

  

  @override
  Widget build(BuildContext context) {
    
    return ListView.builder(
      itemCount: notes.length,
      itemBuilder: (context, index) {
        final note = notes.elementAt(index);
        bool isvisible = isUserAuthor(note.documentId) as bool;
        return ListTile(
          onTap: () {
            onTap(note);
          },
        
          title: Text(
            note.textJob,
            maxLines: 1,
            softWrap: true,
            overflow: TextOverflow.ellipsis,
          ),
          trailing: 
            Visibility(
              visible: isvisible,
            child:  IconButton(
            
            onPressed: () async {
              
              final shouldDelete = await showDeleteDialog(context);
              if (shouldDelete) {
                onDeleteNote(note);
              }
            },
            icon: const Icon(Icons.delete),
          ),)
           
          
        );
      },
    );
  }
}

If you have any other wuestiones to the rest of my code please write me. I don’t programm very long and for that don’t know what else you would need.