fixup! message: Handle disposal of message store correctly

Author: PIG208

lib/model/message.dart

@@ -59,7 +59,10 @@ class MessageStoreImpl with MessageStore {
   }
 
   void dispose() {
-    for (final view in _messageListViews) {
+    // When a MessageListView is disposed, it unregisters itself from the
+    // message store.  We need to iterate over a copy of the set instead to
+    // avoid concurrent modification to the set of registered views.
+    for (final view in _messageListViews.toList()) {
       view.dispose();
     }
   }