title: What does "invalid mode 'kCFRunLoopCommonModes' ..."mean?
body: <p>Environment: Version 11.0 beta 3 (11M362v) </p>
  
  <p>Here's the full text: <br/></p>
  
  <pre><code>invalid mode 'kCFRunLoopCommonModes' provided to CFRunLoopRunSpecific - break on _CFRunLoopError_RunCalledWithInvalidMode to debug. 
  This message will only appear once per execution.
  Hello World
  </code></pre>
  
  <p><p>
  This message only appears when I click on a UISwitch button that is connected to an action; here, printing "Hello World".
  <p>
  Apparently the behavior of the action isn't affected.
  <p>
  <a href="https://i.stack.imgur.com/FmO15.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FmO15.png" alt="enter image description here"></a>
  <p>
  As a comparison, I've created a UIBarButtonItem in the toolbar which behaves normally.  So there's something fishy about the switch button.</p>
  
  <p><strong>Question:</strong> why would this appear and what does it mean?  Remedy?</p>