headerWidget property Null safety
The widget that will be shown above the panel, regardless of PanelState (A persistent widget).
This can be used to drag the panel. (Ofcourse, other parts of the panel will also do).
Generally, a 'pill' or a 'tablet' kind of header can be shown here, that hints user to scroll.
If the height of this widget is not calculatable, it will NOT be shown.
final PanelHeaderWidget headerWidget;