onDragStart property Null safety

AppointmentDragStartCallback? onDragStart

Called whenever the appointment starts to drag in the SfCalendar.

The callback arguments contains the dragging appointment and associated resource details.

Widget build(BuildContext context) {
  return Scaffold(
    body: SfCalendar(
        view: CalendarView.month,
        showWeekNumber: true,
        allowDragAndDrop: true,
        dragAndDropSettings: DragAndDropSettings(
          allowNavigation: true,
          allowScroll: true,
          autoNavigateDelay: Duration(seconds: 1),
          indicatorTimeFormat: 'HH:mm a',
          showTimeIndicator: true,
        onDragStart: (AppointmentDragStartDetails details) {
          dynamic appointment = details.appointment!;
          CalendarResource? resource = details.resource;


final AppointmentDragStartCallback? onDragStart;