didChangeNext method
This route's next route has changed to the given new route.
This is called on a route whenever the next route changes for any reason, so long as it is in the history, including when a route is first added to a Navigator (e.g. by Navigator.push), except for cases when didPopNext would be called.
The nextRoute argument will be null if there's no new next route (i.e.
if isCurrent is true).
Implementation
@override
void didChangeNext(Route<dynamic>? nextRoute) {
  if (nextRoute is ModalRoute<T> &&
      canTransitionTo(nextRoute) &&
      nextRoute.delegatedTransition != delegatedTransition) {
    receivedTransition = nextRoute.delegatedTransition;
  } else {
    receivedTransition = null;
  }
  super.didChangeNext(nextRoute);
  changedInternalState();
}