anchor.diff:  
   class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
     Map<Listenable, VoidCallback>? _handles;
  -  Widget? _child;
  +  late Widget _child;
     bool _keepingAlive = false;
   
     @override
  
positive.code: 
    @override
    Widget build(BuildContext context) {
      assert(_child != null);
      return KeepAlive(
        keepAlive: _keepingAlive,
        child: _child!,
      );
    }
  
  
negative.code:   void _updateChild() {
      _child = NotificationListener<KeepAliveNotification>(
        onNotification: _addClient,
        child: widget.child,
      );
    }
  
  