RestorableRouteFuture<T>  constructor 
    Null safety
- required RoutePresentationCallback onPresent,
- RouteCompletionCallback<T> ? onComplete}
Creates a RestorableRouteFuture.
The onPresent and navigatorFinder arguments must not be null.
Implementation
RestorableRouteFuture({
  this.navigatorFinder = _defaultNavigatorFinder,
  required this.onPresent,
  this.onComplete,
}) : assert(onPresent != null), assert(navigatorFinder != null);