Fix issue: when creating a comment on a list of posts where user never appeared before

lib/lists/users_list.dart

@@ -31,6 +31,9 @@ class UsersList extends ListBase<User> {
     throw "User not found in the list!";
   }
 
+  /// Check if the user is included in this list or not
+  bool hasUser(int userID) => any((f) => f.id == userID);
+
   /// Get the list of users ID present in this list
   List<int> get usersID => List.generate(length, (i) => this[i].id);
 }