The list of observers for the Navigator created for this app.
This list must be replaced by a list of newly-created observers if the navigatorKey is changed.
The Navigator is only built if routes are provided (either via home, routes, onGenerateRoute, or onUnknownRoute); if they are not, navigatorObservers must be the empty list and builder must not be null.
final List<NavigatorObserver>? navigatorObservers;