title: Do not understand error message about NSUnknownKeyException
body: <p>My application is going to crash for a reason I don't know.
  I am getting an error like this: </p>
  
  <blockquote>
    <p>" Terminating app due to uncaught exception 'NSUnknownKeyException',
    reason: '[ setValue:forUndefinedKey:]:
    this class is not key value coding-compliant for the key
    viewController."</p>
  </blockquote>