title: Undecorate JInternalFrame on MacOS
body: <p>I am trying to undecorate a <code>JInternalFrame</code>, i.e. remove the default titlebar using the following code;</p>
  
  <pre><code>BasicInternalFrameUI ui = (BasicInternalFrameUI)internalFrame.getUI();
  ui.getNorthPane().setPrefrredSize(new Dimension(0,0));
  </code></pre>
  
  <p>I works on windows but the second line throws a NullPointerException on MacOS</p>
  
  <p>Any ideas why and how to get round it?</p>