fix(flutter): fix concurrent modification error when closing socket

Since subscription.close() modifies _subscriptions, we need to create a
separate list to avoid concurrent modification error.

Author: stnguyen90

templates/flutter/lib/src/realtime_mixin.dart.twig

@@ -87,8 +87,8 @@ mixin RealtimeMixin {
             break;
         }
       }, onDone: () {
-        if (!_notifyDone || _creatingSocket) return;
-        for (var subscription in _subscriptions.values) {
+        final subscriptions = List.from(_subscriptions.values);
+        for (var subscription in subscriptions) {
           subscription.close();
         }
         _channels.clear();