The widget that can be "opened" with the CupertinoContextMenu.
When the CupertinoContextMenu is long-pressed, the menu will open and this widget (or the widget returned by previewBuilder, if provided) will be moved to the new route and placed inside of an Expanded widget. This allows the child to resize to fit in its place in the new route, if it doesn't size itself.
This parameter cannot be null.
final Widget child