autocomplete: Break search loop when _sortedUsers invalidated

lib/model/autocomplete.dart

@@ -348,12 +348,19 @@ class MentionAutocompleteView extends ChangeNotifier {
       _sortUsers();
     }
 
-    final iterator = _sortedUsers!.iterator;
+    final sortedUsers = _sortedUsers!;
+    final iterator = sortedUsers.iterator;
     bool isDone = false;
     while (!isDone) {
       // CPU perf: End this task; enqueue a new one for resuming this work
       await Future(() {});
 
+      if (_sortedUsers != sortedUsers) {
+        // The list of users this loop has been working from has become stale.
+        // Abort so _startSearch can retry with the new list.
+        throw ConcurrentModificationError();
+      }
+
       if (query != _query || !hasListeners) { // false if [dispose] has been called.
         return null;
       }