Flutter 报错 setState() called after dispose()

if(mounted){
       setState(() {
           ........
       });
     }
    });

 /// Whether this [State] object is currently in a tree.
  ///
  /// After creating a [State] object and before calling [initState], the
  /// framework "mounts" the [State] object by associating it with a
  /// [BuildContext]. The [State] object remains mounted until the framework
  /// calls [dispose], after which time the framework will never ask the [State]
  /// object to [build] again.
  ///
  /// It is an error to call [setState] unless [mounted] is true.
  bool get mounted => _element != null;