Fix automatic keep alive.

Author: Yuncong Zhang

Runtime/widgets/automatic_keep_alive.cs

@@ -67,6 +67,9 @@ bool _addClient(KeepAliveNotification notification) {
                 }
                 else {
                     SchedulerBinding.instance.addPostFrameCallback(timeStamp => {
+                        if (!this.mounted) {
+                            return;
+                        }
                         ParentDataElement childElement1 = this._getChildElement();
                         D.assert(childElement1 != null);
                         this._updateParentDataOfChild(childElement1);