anchor.diff:  
     @override
     Widget build(BuildContext context) {
  -    assert(_child != null);
       return KeepAlive(
         keepAlive: _keepingAlive,
  -      child: _child!,
  +      child: _child,
       );
     }
   
  
positive.code:   /// [KeepAlive] widget appropriately.
    const AutomaticKeepAlive({
      Key? key,
      this.child,
    }) : super(key: key);
  
    /// The widget below this widget in the tree.
    ///
    /// {@macro flutter.widgets.ProxyWidget.child}
    final Widget? child;
  
    @override
    State<AutomaticKeepAlive> createState() => _AutomaticKeepAliveState();
  
negative.code:   void _updateChild() {
      _child = NotificationListener<KeepAliveNotification>(
        onNotification: _addClient,
        child: widget.child,
      );
    }
  
  