restorationScopeId property Null safety
The restoration scope ID to be used for the RestorationScope surrounding this route.
The restoration scope ID is null if restoration is currently disabled for this route.
If the restoration scope ID changes (e.g. because restoration is enabled or disabled) during the life of the route, the ValueListenable notifies its listeners. As an example, the ID changes to null while the route is transitioning off screen, which triggers a notification on this field. At that point, the route is considered as no longer present for restoration purposes and its state will not be restored.
ValueListenable<String?> get restorationScopeId => _restorationScopeId;