Xibs instead of StoryboardsAppDelegate.m

Non Universal (iPhone OR iPad)

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    MainViewController *mainViewController  = [[MainViewController alloc] init];
    self.window.rootViewController = mainViewController;
    [self.window makeKeyAndVisible];
    return YES;
}

Universal (iPhone AND iPad)

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    
    // assumes the nib names are "RootViewController~iPhone.xib and "RootViewController~iPad.xib"
    RootViewController *rootViewController = [[RootViewController alloc] init];
    self.window.rootViewController = rootViewController;
    [self.window makeKeyAndVisible];
    return YES;
}

With Navigation Controller

@interface AppDelegate()

@property (strong, nonatomic) UINavigationController *navigationController;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    
    // assumes the nib names are "RootViewController~iPhone.xib and "RootViewController~iPad.xib"
    RootViewController *rootViewController = [[RootViewController alloc] init];    
    self.navigationController = [[UINavigationController alloc] initWithRootViewController:rootViewController];
    [self.window setRootViewController:self.navigationController];
    
    [self.window makeKeyAndVisible];
    return YES;
}