A key to use when building the Navigator.
If a navigatorKey is specified, the Navigator can be directly manipulated without first obtaining it from a BuildContext via Navigator.of: from the navigatorKey, use the GlobalKey.currentState getter.
If this is changed, a new Navigator will be created, losing all the application state in the process; in that case, the navigatorObservers must also be changed, since the previous observers will be attached to the previous navigator.
The Navigator is only built if onGenerateRoute is not null; if it is null, navigatorKey must also be null.
final GlobalKey<NavigatorState>? navigatorKey;