anchor.diff:    static const String routeName = '/list';
   
     @override
  -  ListDemoState createState() => new ListDemoState();
  +  _ListDemoState createState() => new _ListDemoState();
   }
   
  -class ListDemoState extends State<ListDemo> {
  +class _ListDemoState extends State<ListDemo> {
     static final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();
   
     PersistentBottomSheetController<Null> _bottomSheet;
  
positive.code:     _bottomSheet?.setState(() { });
    }
  
    void showConfigurationSheet(BuildContext appContext) {
      _bottomSheet = scaffoldKey.currentState.showBottomSheet((BuildContext bottomSheetContext) {
        return new Container(
          decoration: new BoxDecoration(
            border: new Border(top: new BorderSide(color: Colors.black26)),
  
negative.code:           ],
          ),
        );
      });
  
      setState(() {
        _bottomSheet = bottomSheet;
      });
  
      _bottomSheet.closed.whenComplete(() {
        if (mounted) {
          setState(() {
            _bottomSheet = null;
          });
        }
      });
    }
  
  