Restoration ID to save and restore the state of the CupertinoTabScaffold.
This property only has an effect when no controller has been provided: If it is non-null (and no controller has been provided), the scaffold will persist and restore the currently selected tab index. If a controller has been provided, it is the responsibility of the owner of that controller to persist and restore it, e.g. by using a RestorableCupertinoTabController.
- RestorationManager, which explains how state restoration works in Flutter.
final String? restorationId;