应用程序打开到空白黑屏

我正在制作一个将导航控制器嵌入到 tabbarcontroller 的应用程序.现在，当我打开应用程序时，我只看到一个空白的黑屏.

I am making an app that embeds a navigationcontroller into a tabbarcontroller. Now when I open the app I am getting just a blank black screen.

这是我的代码

PDCFirstViewController *viewController1 = [[PDCFirstViewController alloc] 
initWithNibName:@"PDCFirstViewController" bundle:nil];

PDCSecondViewController *viewController2 = [[PDCSecondViewController alloc] 
initWithNibName:@"PDCSecondViewController" bundle:nil];

ViewController *viewController3 = [[ViewController alloc] 
initWithNibName:@"ViewController" bundle:nil];
UINavigationController *navigationcontroller = [[UINavigationController alloc] 
initWithRootViewController:viewController3];

self.tabBarController = [[UITabBarController alloc] init];
self.tabBarController.viewControllers = [NSArray  
arrayWithObjects:viewController1,viewController2,navigationcontroller, nil];

[self.window makeKeyAndVisible];

我需要添加一些东西或做一些不同的事情来让应用程序显示吗?任何帮助都会很棒！谢谢！

Do I need to add something or do something different to make the app display? Any assistance would be great! Thank you!