title: Why can't I use pushViewController with UIButton?
body: <p>I want to use pushViewController but I can't. I remember that I can use this code in xcode3.</p>
  
  <p>Here is  code in IBAction Button </p>
  
  <pre><code>-(IBAction)changtoCal:(id)sender
  {
      NSLog(@"get in");
      UINavigationController *cal_nav = [[UINavigationController alloc] initWithNibName:@"calculator" bundle:nil];
      [self.navigationController pushViewController:cal_nav animated:YES];
  }
  </code></pre>
  
  <p>It don't push. Why ??? Could you tell me and how can I push it ???</p>
  
  <p>And here is my AppDelegate 
  .h</p>
  
  <pre><code>@class ViewController;
  
  @interface AppDelegate : UIResponder &lt;UIApplicationDelegate&gt;
  {
      UINavigationController *navigationController;
  }
  
  @property (strong, nonatomic) UIWindow *window;
  
  @property (strong, nonatomic) ViewController *viewController;
  
  @property (strong, nonatomic) IBOutlet UINavigationController *navigationController;
  
  @end
  </code></pre>
  
  <p>.m</p>
  
  <pre><code>- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
  {
      self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
      self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
      self.window.rootViewController = self.viewController;
      [_window addSubview:[navigationController view]];
      [self.window makeKeyAndVisible];
      return YES;
  }
  </code></pre>