xcode11创建新项目nav+tab无效

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
   
    FirstViewController *first = [FirstViewController new];
    UINavigationController *nav1 = [[UINavigationController alloc] initWithRootViewController:first];
    
    UITabBarItem *item1 = [[UITabBarItem alloc]init];
    item1.title = @"tab1";
    item1.image = [UIImage imageNamed:@"iPhone_患者端首页_医院_未选中"];
    nav1.tabBarItem = item1;
    
    SecondViewController *second = [SecondViewController new];
    UINavigationController *nav2 = [[UINavigationController alloc] initWithRootViewController:second];
    
    UITabBarItem *item2 = [[UITabBarItem alloc]init];
    item2.title = @"tab2";
    item2.image = [UIImage imageNamed:@"iPhone_患者端首页_医院_未选中"];
    nav2.tabBarItem = item2;
    
    UITabBarController *tab = [[UITabBarController alloc] init];
    tab.viewControllers = @[nav1,nav2];
    
    
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    [self.window setBackgroundColor:[UIColor whiteColor]];
    
    self.window.rootViewController = tab;
    [self.window makeKeyAndVisible];
    
    return YES;
}

//- (UISceneConfiguration *)application:(UIApplication *)application configurationForConnectingSceneSession:(UISceneSession *)connectingSceneSession options:(UISceneConnectionOptions *)options {
//    // Called when a new scene session is being created.
//    // Use this method to select a configuration to create the new scene with.
//    return [[UISceneConfiguration alloc] initWithName:@"Default Configuration" sessionRole:connectingSceneSession.role];
//}
//
//
//- (void)application:(UIApplication *)application didDiscardSceneSessions:(NSSet<UISceneSession *> *)sceneSessions {
//    // Called when the user discards a scene session.
//    // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
//    // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
//}