我升级到iOS9，开始出现这个错误。我能够修复它，但添加以下代码到- (BOOL)应用程序:(UIApplication *)应用程序didFinishLaunchingWithOptions:(NSDictionary *)launchOptions

NSArray *windows = [[UIApplication sharedApplication] windows];
for(UIWindow *window in windows) {
    if(window.rootViewController == nil){
        UIViewController* vc = [[UIViewController alloc]initWithNibName:nil bundle:nil];
        window.rootViewController = vc;
    }
}

选择你的“窗口”在你的Nib文件 在“属性检查器”中选择“启动时可见”

手动创建nib文件时会发生这种情况。 这个修复工作于常规的nib模式-不是故事板模式

我收到了同样的错误信息，因为我调用了警报

- (void)applicationDidBecomeActive:(UIApplication *)application 

而不是

- (void)applicationWillEnterForeground:(UIApplication *)application

OrdoDei给出了一个正确而有价值的答案。我添加这个答案只是为了给出一个didFinishLaunchingWithOptions方法的例子，该方法使用了他的答案以及其他关于导航控制器的评论。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    // Override point for customization after application launch.

    // Instantiate the main menu view controller (UITableView with menu items).
    // Pass that view controller to the nav controller as the root of the nav stack.
    // This nav stack drives our *entire* app.
    UIViewController *viewController = [[XMMainMenuTableViewController alloc] init];
    self.navigationController = [[UINavigationController alloc] initWithRootViewController:viewController];

    // Instantiate the app's window. Then get the nav controller's view into that window, and onto the screen.
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // [self.window addSubview:self.navigationController.view];
    // The disabled line above was replaced by line below. Fixed Apple's complaint in log: Application windows are expected to have a root view controller at the end of application launch
    [self.window setRootViewController:self.navigationController];
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
}

在升级到Xcode 4.3后，我开始遇到同样的问题，而且只有在从头开始一个项目时(即创建一个空项目，然后创建一个UIViewController，然后创建一个单独的nib文件)。

在把我习惯的所有行，并确保我有正确的连接后，我一直得到这个错误，我试图通过视图控制器加载的nib文件(它被设置为rootController)从未在模拟器中显示。

我通过Xcode创建了一个视图模板，并将其与我的代码进行了比较，最终发现了问题!

Xcode 4.3 appears to add by default the method -(void)loadView; to the view controller implementation section. After carefully reading the comments inside it, it became clear what the problem was. The comment indicated to override loadView method if creating a view programmatically (and I'm paraphrasing), otherwise NOT to override loadView if using a nib. There was nothing else inside this method, so in affect I was overriding the method (and doing nothing) WHILE using a nib file, which gave the error.

解决方案是从实现部分完全删除loadView方法，或者通过添加[super loadView]来调用父方法。

如果使用NIB文件，则最好删除它，因为添加任何其他代码都会覆盖它。

当没有正确设置接口生成器时，会发生此问题。

确保你的App Delegate的窗口和viewController outlet被连接起来:

在你的主窗口。xib，按住control，点击App Delegate并拖动到Window对象。选择窗口。按住control并再次选择App委托，拖到你的根视图控制器并选择viewController。