I have followed the instructions but cannot get this to work. I have added the code to Page.php as below

<?php

class Page extends SiteTree {
   
   public static $db = array(

   );
   
   public static $has_one = array(

   );
   
   function onBeforeWrite() {
$this->Content = preg_replace('|<iframe(.*)/>|Uims', '<iframe\\1> </iframe>', $this->Content);
parent::onBeforeWrite();
}
   public function getCMSFields()
   {
   $fields = parent::getCMSFields();   

   //Remove Reports/todo tabs
   $fields->removeByName("To-do");
   $fields->removeByName("Reports");

      return $fields;
   }   
}

but I just get an error when I do a dev/build?

Can anybody help?