dragDetails property Null safety
If the Scrollable stopped scrolling because of a drag, the details about that drag end.
If a drag ends with some residual velocity, a typical ScrollPhysics will start a ballistic scroll, which delays the ScrollEndNotification until the ballistic simulation completes, at which time dragDetails will be null. If the residual velocity is too small to trigger ballistic scrolling, then the ScrollEndNotification will be dispatched immediately and dragDetails will be non-null.
final DragEndDetails? dragDetails;