title: BeginSheet doesn't show window
body: <p>I have simple code : </p>
  
  <pre><code>var openPanel = NSOpenPanel();
  openPanel.beginSheet(this.View.Window, (obj) =&gt; {
      //do stuff
      openPanel.endSheet(this.View.Window);
  });
  </code></pre>
  
  <p>Sometime the sheet window is not shown and makes a sound like the window is busy. Is there anything wrong in my code?</p>
  
  <p>I call this code from one item of splitViewcontroller.</p>