获取“应用程序窗口应在应用程序启动结束时具有根视图控制器”错误

-(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions  {

    {
        CGSize iosScreenSize = [[UIScreen mainScreen] bounds].size;

        if (iosScreenSize.height == 667) {

            UIStoryboard *iPhone6 = [UIStoryboard storyboardWithName:@"iPhone6" bundle:nil];

            UIViewController *initialViewController =[iPhone6 instantiateInitialViewController];

            self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen]bounds]];

            self.window.rootViewController = initialViewController;

            [self.window makeKeyAndVisible];

        }

        return YES;
    }

    return YES;
}
-(BOOL)application:(UIApplication *)application