anchor.diff:      } else {
         _tabController.index = 1;
       }
  +  }
  +
  +  void dispose() {
  +    _tabController.dispose();
  +    super.dispose();
     }
   
     @override
  
positive.code: class AllBooksScreen extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return Center(
        child: Text('All Books'),
      );
    }
  }
  
negative.code:   @override
    final GlobalKey<NavigatorState> navigatorKey;
    final AppState _appState = AppState();
    late final OverlayEntry _overlayEntry =
    OverlayEntry(builder: _overlayEntryBuilder);
  
    BookRouterDelegate() : navigatorKey = GlobalKey<NavigatorState>() {
      _appState.addListener(handleAppStateChanged);
    }
  
    void handleAppStateChanged() {
      notifyListeners();
      _overlayEntry.markNeedsBuild();
    }
  
    @override
  