title: UIBarButtonItem not appearing
body: <p>My app layout is as follows -</p>
  
  <p>the rootViewController is a tabViewController with 3 tabs each having a UINavigationController as their rootViewController. Within one of these tabs I am pushing upon cell selection to another tabController which now has two tabs. What I am trying to do is set the rightBarButtonItem on each of these two tab's viewControllers... in the viewDidLoad method of both of these I am doing:</p>
  
  <pre><code>self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemEdit target:self action:@selector(selectionChanged:)];
  </code></pre>
  
  <p>however this is doing absolutely nothing! I thought from the apple documentations that you could set the navigationItem's rightBarButtonItem from anywhere within your navigation controllers view hierarchy but that doesn't seem to be the case here. Any idea what - if anything  - I am doing wrong?</p>