createRectTween property Null safety
Defines how the destination hero's bounds change as it flies from the starting route to the destination route.
A hero flight begins with the destination hero's child aligned with the starting hero's child. The Tween<Rect> returned by this callback is used to compute the hero's bounds as the flight animation's value goes from 0.0 to 1.0.
final CreateRectTween? createRectTween;