title: @IBDesignable crashing agent
body: <p>When I write my own <code>UIButton</code>-extended class and make it <code>@IBDesignable</code>, I receive two errors in Interface Builder, namely:</p>
  
  <ul>
  <li>Main.storyboard: error: IB Designables: Failed to update auto layout status: The agent crashed because the fd closed</li>
  <li>Main.storyboard: error: IB Designables: Failed to render instance of RandjeUIButton: The agent crashed</li>
  </ul>
  
  <p>Here is my code:</p>
  
  <pre><code>import UIKit
  
  @IBDesignable
  class RandjeUIButton: UIButton {
      required init(coder aDecoder: NSCoder) {
          super.init(coder: aDecoder)
  
          self.backgroundColor = UIColor.blackColor()
      }
  }
  </code></pre>
  
  <p>I am working in Xcode 7 beta 2 on OS X 10.11 beta 2. (Running in VM)</p>