onExit method
Describes how transitions from this state should be handled.
The build function is called with a TransitionHandlerBuilder that can be used to describe
the behavior of the exit transition.
Implementation
void onExit(
void Function(TransitionHandlerBuilder<D, void>) build,
) {
var builder = TransitionHandlerBuilder<D, void>._(key, _log, _makeVoidTransitionContext);
build(builder);
_onExit = builder._descriptor;
}