getMentionedUsers method
Implementation
List<User> getMentionedUsers(String text) {
  List<User> mentionedUsers = [];
  for (List<User?> users in mentionedUsersMap.values) {
    for (var user in users) {
      if (user != null &&
          mentionedUsers.indexWhere(
                (element) => element.uid == user.uid,
              ) ==
              -1) {
        mentionedUsers.add(user);
      }
    }
  }
  return mentionedUsers;
}