anchor.diff:    /// [KeepAlive] widget appropriately.
     const AutomaticKeepAlive({
       Key? key,
  -    this.child,
  +    required this.child,
     }) : super(key: key);
   
     /// The widget below this widget in the tree.
     ///
     /// {@macro flutter.widgets.ProxyWidget.child}
  -  final Widget? child;
  +  final Widget child;
   
     @override
     State<AutomaticKeepAlive> createState() => _AutomaticKeepAliveState();
  
positive.code:   void _updateChild() {
      _child = NotificationListener<KeepAliveNotification>(
        onNotification: _addClient,
        child: widget.child!,
      );
    }
  
  
negative.code: 
  class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
    Map<Listenable, VoidCallback>? _handles;
    late Widget _child;
    bool _keepingAlive = false;
  
    @override
  