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: 
    @override
    Widget build(BuildContext context) {
      assert(_child != null);
      return KeepAlive(
        keepAlive: _keepingAlive,
        child: _child!,
      );
    }
  
  
negative.code: 
  class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
    Map<Listenable, VoidCallback>? _handles;
    late Widget _child;
    bool _keepingAlive = false;
  
    @override
  