title: Failed to connect (storyboard) outlet from (NSApplication) to (NSNibExternalObjectPlaceholder) error in Cocoa and storyboard
body: <p>I've tried to build a sample Cocoa app on which I want to connect UI components put on storyboard to <code>ViewController.swift</code> as either an <code>IBOutlet</code> or <code>IBAction</code>. However, when I tried to control-drag the UI components on storyboard (such as <code>NSButton</code>) to <code>ViewController.swift</code> and create a <code>@IBAction</code> method, and then run the app, the resultant app logs the following message in console and of course the app doesn't respond to me tapping the button.</p>
  
  <pre><code>Failed to connect (storyboard) outlet from (NSApplication) to (NSNibExternalObjectPlaceholder): missing setter or instance variable
  </code></pre>
  
  <p>How can I use the <code>IBAction</code> method properly?</p>
  
  <p>For your information here's my <code>ViewController.swift</code>:</p>
  
  <pre><code>import Cocoa
  
  class ViewController: NSViewController {
      @IBOutlet var txtTitle : NSTextField
      @IBOutlet var boxColor : NSBox
  
      override func viewDidLoad() {
          super.viewDidLoad()
      }
  
      func colorChanged(cp: NSColorPanel) {
          let c:NSColor = cp.color;
          self.boxColor.fillColor = c
      }
  
      @IBAction func btnSetColor(sender : AnyObject) {
          let cp:NSColorPanel = NSColorPanel.sharedColorPanel()
          cp.setTarget(self)
          cp.setAction("colorChanged:")
          cp.orderFront(nil)
  
      }
  
      @IBAction func btnSetWindowTitle(sender : AnyObject) {
          if self.txtTitle.stringValue != "" {
              println(self.title)
              println(self.txtTitle.stringValue)
              self.title = self.txtTitle.stringValue
          }
      }
  }
  </code></pre>
  
  <p>I use Xcode 6 beta on OS X 10.10 Yosemite. And started the template with storyboard being on.</p>