title: Xcode Playground Timeline empty
body: <p>I am trying to use the timeline assistant editor for playground in xcode version 7.3.1, it is always empty.
  <a href="http://i.stack.imgur.com/pGQeh.png" rel="noreferrer">Timeline assistant editor</a></p>
  
  <p>I think the error is from xcode, however from search it doesn't look like anyone got the same error so i am confused.</p>