мои роуты выглядят несколько иначе
Routes.navigatorKey.currentState.pushReplacementNamed(action.payload.first,arguments: action.payload.second);
и вот так
static Route onGenerateRouteDelegate(RouteSettings settings) {
….
CupertinoPageRoute(
fullscreenDialog: true,
settings: RouteSettings(name:
settings.name),
builder: (context) => Material(child: LoginPage(login: settings?.arguments ?? "7777")),
);