title: Xcode 9 update Swift, purple warning
body: <p>Not sure why after updating to Xcode 9 my project (originally created in Swift 3) is showing purple warnings:
  <strong><em>UITextField.text must be used from main thread only</em></strong></p>
  
  <p>I'm only checking in If statement if text field is empty... attaching screenshot. </p>
  
  <p><a href="https://i.stack.imgur.com/fwTWK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fwTWK.png" alt="enter image description here"></a></p>