applications are expected to have a root view controller at the end of application launch

If you met this error when you developing ios app, you can refer to this link.

Basically, 2 steps.

1st step, add a RootViewController property in AppDelegate.h.

Add this line before AppDelegate declaration.

@class RootViewController;
Add this line after AppDelegate declaration.
@property (nonatomic, retain) RootViewController *rootViewController;

2nd step, init a RootViewController instance and assign it to the window's root view controller, in didFinishLaunchingWithOptions method.

_rootViewController = [[RootViewController alloc] init];
[[self window] setRootViewController: [self rootViewController]];

After adding, it would be like this:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.backgroundColor = [UIColor whiteColor];
    _rootViewController = [[RootViewController alloc] init];
    [[self window] setRootViewController: [self rootViewController]];
    [self.window makeKeyAndVisible];
    return YES;
}