title: Swift: Disable or Clear Undo for a TextField
body: <p>When selecting NumberPad for a TextField, the iPad displays a full keyboard, so I have created a function to simply remove all characters that are not numeric.  However, if a Non-numeric key is pressed and then the user presses Undo on the keyboard, the App Crashes.</p>
  
  <p>How do I disable the Undo function for the TextField or at least clear the Undo Stack?</p>