title: Unchecked runtime.lastError when using Chrome API
body: <p>I use <code>chrome.fileSystem</code> API in my app to open a file. When I click the <code>Cancel</code> button of the file chooser dialog, an error occurs:</p>
  
  <blockquote>
    <p>Unchecked <code>runtime.lastError</code> while running <code>fileSystem.chooseEntry</code>: User cancelled</p>
  </blockquote>
  
  <p>How to fix this error?</p>