fix leak on demo

example/lib/send_auth.dart

@@ -21,6 +21,12 @@ class _SendAuthPageState extends State<SendAuthPage> {
     });
   }
 
+  @override
+  void dispose() {
+    super.dispose();
+    _result = null;
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(