title: NSButton does not call its action
body: <p>I am having a little trouble with NSButton's action. I programmatically create a button ...</p>
  
  <pre><code>let continueButton = NSButton(title: "Continue", target: self, action: #selector(self.continueButtonClicked))
  </code></pre>
  
  <p>... to call this function:</p>
  
  <pre><code>@objc func continueButtonClicked() {
      print("continueButtonClicked")
  }
  </code></pre>
  
  <p>However, <code>continueButtonClicked</code> is never called and the button appears to be disabled. I've tried enabling it, but that didn't do the trick.</p>
  
  <p>Am I missing something?</p>