showMenu function in flutter

 class _SettingsPopupMenuState extends State<SettingsPopupMenu> {
      static const Map<String, IconData> _options = {
       'Settings' : Icons.favorite_border,
       'Share'    : Icons.bookmark_border,
       'Logout'   : Icons.share,
      };

      void _showPopup(BuildContext context) async {
        //*get the render box from the context
        final RenderBox renderBox = context.findRenderObject() as RenderBox;
        //*get the global position, from the widget local position
        final offset = renderBox.localToGlobal(Offset.zero);

        //*calculate the start point in this case, below the button
        final left = offset.dx;
        final top = offset.dy + renderBox.size.height;
        //*The right does not indicates the width
        final right = left + renderBox.size.width;

        //*show the menu
        final value = await showMenu<String>(
          // color: Colors.red,
          shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(20.0)
          ),
          context: context, 
          position: RelativeRect.fromLTRB(left, top, right, 0.0), 
          items: _options.entries.map<PopupMenuEntry<String>>((entry) {
            return PopupMenuItem(
              value: entry.key,
              child: SizedBox(
                // width: 200, //*width of popup
                child: Row(
                  children: [
                    Icon(entry.value, color: Colors.redAccent),
                    const SizedBox(width: 10.0),
                    Text(entry.key)
                  ],
                ),
              ),
            );
          }).toList()
        );

        print(value);
      }

      @override
      Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(
          title: const Text('Popup Settings'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              //*encloses the widget from which the relative positions will be 
              //*taken in a builder, in this case a button
              Builder(
                builder: (context) {
                  return RawMaterialButton(
                    fillColor: Colors.indigo,
                    constraints: const BoxConstraints(minWidth: 200),
                    onPressed: () => _showPopup(context),
                    shape: RoundedRectangleBorder(
                      borderRadius: BorderRadius.circular(8.0)
                    ),
                    padding: const EdgeInsets.symmetric(vertical: 10.0),
                    child: const Text('Show Menu', style: TextStyle(color: 
    Colors.white)),
                  );
                }
              ),
            ],
          ),
        ),
      );
    }
  }