Can't Push Nav Controller

gistfile1.m

//
//  AppDelegate.m
//  fhwei
//
//

#import "AppDelegate.h"
@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    
    UIViewController * vc = [[UIViewController alloc] init];
    vc.view.backgroundColor = [UIColor greenColor];
    UINavigationController * nav = [[UINavigationController alloc ] initWithRootViewController:vc];

    
    UINavigationController * mainNav = [[UINavigationController alloc] init];
    
    [mainNav pushViewController:nav animated:NO];
    //[mainNav setViewControllers:@[nav]];
    
    [self.window setRootViewController:mainNav];
    [self.window makeKeyAndVisible];
    return YES;
}



- (UIWindow *)window
{
    if(_window == nil)
        _window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    return _window;
}



@end