Cancel route navigation in onGenerateRoute

Is there any official way to cancel a route navigation from the onGenerateRoute method?

Right now I've "solved" it by just returning null, which kind of works but generates error messages in the log.

Route _onGenerateRoute(RouteSettings settings) {
  Widget page =
    settings.name == 'page1' ? Page1() :
    settings.name == 'page2' ? Page2() :
      null;

  if( page == null ) return null;

  return MaterialPageRoute(
    builder: (ctx) => page,
    settings: settings
  );
}

So I'm wondering if there is a proper way to cancel a navigation - and just stay on the current route - from onGenerateRoute?