Called when onGenerateRoute fails to generate a route, except for the initialRoute.
This callback is typically used for error handling. For example, this callback might always generate a "not found" page that describes the route that wasn't found.
Unknown routes can arise either from errors in the app or from external requests to push routes, such as from Android intents.
The Navigator is only built if routes are provided (either via home, routes, onGenerateRoute, or onUnknownRoute); if they are not, builder must not be null.
final RouteFactory? onUnknownRoute;