title: Drupal shows "node_form_validate() undefined" error while trying to create a poll
body: <p>I have a Drupal 7 website and would like to add a poll to it. I have enabled a Poll module and opened Add material -> Poll page. When I pressed "add variant" button I got an error stating that </p>
  
  <pre><code>AJAX HTTP error.
  HTTP response code: 200
  Debugging information follows.
  Path: /system/ajax
  Text status: OK
  ResponseText: 
  Fatal error:  Call to undefined function node_form_validate() in /*/public_html/includes/form.inc on line 1390
  </code></pre>
  
  <p>How can it be fixed? I have rather small experience with Drupal and hardly imagine where to start.</p>