title: Add UINavigationController inside UIViewController
body: <p>I have a UIViewController with a UIToolbar (on bottom) and I want to add a UINavigationController with UINavigationBar inside.
  But the UINavigationController is not displayed.</p>
  
  <p><strong>MyViewController.m :</strong></p>
  
  <pre><code>- (void)viewDidLoad
  {
      [super viewDidLoad];
  
      int toolBarHeight = 44;
      UIToolbar *toolBar = [[UIToolbar alloc] initWithFrame:CGRectMake(0, [self.view bounds].size.height-toolBarHeight, [self.view bounds].size.width, toolBarHeight)];
  
      UIBarButtonItem *button = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemRefresh target:nil action:nil];
      toolBar.items = @[button];
  
      [self.view addSubview:toolBar];
  
      MyNavigationController *myNav = [[MyNavigationController alloc] init];
  
      [self addChildViewController:myNav];
  }
  </code></pre>