title: Selected BarButtonItem not showing.
body: <p>Whenever the user selects the <code>UIViewController</code> the selected <code>BarButtonItem</code> disappears. In apple's documentation it recommends using two separate images for the selected state, and the unselected state. I do this, but the the selected image does not show up. In the console I get the following code. How can I make it so that when the selected image shows up.<br><br> 
  <code>2015-01-03 10:55:22.718 Grouper[2105:130730] CUICatalog: Invalid asset name supplied:           (null)
  2015-01-03 10:55:22.718 Grouper[2105:130730] CUICatalog: Invalid asset name supplied: (null)
  2015-01-03 10:55:22.718 Grouper[2105:130730] Could not load the "(null)" image referenced from a nib in the bundle with identifier "SR.Grouper"</code><br><br>If the answer involves code, please use swift, as I am not very familiar with objective-c.</p>
  
  <p><img src="https://i.stack.imgur.com/5uQbN.png" alt="enter image description here"></p>