anchor.diff:  /// Note that if a mouse has never entered, it will not remove itself.
   class HoverCard {
     HoverCard({
  -    @required BuildContext context,
  -    @required Widget contents,
  -    @required double width,
  -    @required Offset position,
  -    String title,
  +    required BuildContext context,
  +    required Widget contents,
  +    required double width,
  +    required Offset position,
  +    String? title,
     }) {
  -    final overlayState = Overlay.of(context);
  +    final overlayState = Overlay.of(context)!;
       final colorScheme = Theme.of(context).colorScheme;
       final focusColor = Theme.of(context).focusColor;
       final hoverHeading = colorScheme.hoverTitleTextStyle;
  
positive.code:   final Widget child;
  
    /// Disposable object to be disposed when the group is closed.
    final Disposable disposable;
  
    @override
    _HoverCardTooltipState createState() => _HoverCardTooltipState();
  
negative.code:           title: title,
          );
  
    late OverlayEntry _overlayEntry;
  
    bool _isRemoved = false;
  
  