UIWindow rootViewController couldn't find UIViewController subclass

Question

I have a CSRootViewController:UIViewController without xib file, a CSFirstViewController:CSRootViewController with xib file. For bellow code I will get the error:

"Application windows are expected to have a root view controller at the end of application launch"

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

    CSFirstViewController *firstViewController = [[CSFirstViewController alloc] init];
    self.window.rootViewController = firstViewController;
    [self.window makeKeyAndVisible];

    return YES;
}

update

Here is the CSFirstViewController'code. .h file:

@interface CSFirstViewController : CSRootViewController

@end

And the .m file

@interface CSFirstViewController ()

- (IBAction)pushButtonClicked:(id)sender;

@end

@implementation CSFirstViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - IBAction methods

- (IBAction)pushButtonClicked:(id)sender
{
    CSSecondViewController *secondVC = [[CSSecondViewController alloc] init];
    [self.rootViewController pushCSViewController:secondVC animated:YES];
}

@end

Any guys could point me out the reason?

Answer 1

I find the way to handle my question. Just add [self.window addSubview:firstViewController.view]; after set window's rootViewController. But I still don't know why this happens.