title: LeftBarButtonItem and RightBarButtonItem not shown in MonoTouch.Dialog DialogViewController
body: <p>MonoTouch is 5.2.12. iOS is v5.1.1 (Simulator)</p>
  
  <p>Seems like I'm missing an important piece of a puzzle here.
  I'm subclassing <code>DialogViewController</code> and set it as the only controller in my UINavigationController.
  In <code>ViewWillAppear</code> of the subclassed <code>DialogViewController</code>, I'm trying to set the left and right bar button items:</p>
  
  <pre><code>this.NavigationController.NavigationItem.LeftBarButtonItem = new UIBarButtonItem(UIBarButtonSystemItem.Done, this.HandleDoneButtonTouched );
  </code></pre>
  
  <p>Neither one of them is showing up. The title however is displayed. If I debug, I can see that the items are properly set.</p>
  
  <p>I have also tried to use <code>SetItems()</code> on the navigation controller: no effects either.</p>
  
  <p>The navigation controller is presented modally in a page sheet.</p>