title: Binding from tabbed view to SharedUserDefaultsController proxy makes view unusable
body: <p>I need to create the GUI for setting my app preferences. Since I am not able to get multiple instances of the SharedUserDefaultsController in multiple storyboard scenes, I created a simple proxy for it. I'm trying to bind a text field content to a specific values of the SharedUserDefaultsController through the proxy which is represented in IB via an NSObject added to the scene that should contain the binding. For the binding I use self.defaults.values.default_key where default_key is a defined key in the StandardUserDefaults for my application.</p>
  
  <p>When I try to show the view that contains the binding, though, it is not shown and the only displayed message in the console is</p>
  
  <blockquote>
    <p>Missing placeholder for identifier UpstreamPlaceholder-37m-v1-XT6</p>
  </blockquote>
  
  <p>What am I doing wrong? In the storyboard source there's no trace of that identifier. Here's the code of the SharedUserDefaultsControllerProxy class that I created:</p>
  
  <pre><code>import Cocoa
  
  class SharedUserDefaultsControllerProxy: NSObject{
      var defaults = NSUserDefaultsController.sharedUserDefaultsController()
  
      override init() {
          super.init()
      }
  
  }
  </code></pre>