title: Unable to use Undo in TextChanged
body: <p>When using textbox.Undo(); I get the following error:</p>
  
  <blockquote>
    <p>Cannot Undo or Redo while undo unit is open.</p>
  </blockquote>
  
  <p>Now I understand why this is the case (because it is an active undoable event) but through what event can I perform validation on a text box and undo the change if the user has typed an invalid character?</p>