Correggere Themler Error 500 in editor, dopo aggiornamento Joombla

Fatal error: require_once(): Failed opening required '[...]/administrator/includes/toolbar.php' (include_path='.:/usr/lib/php7.2') in [...]/templates/[theme_name]/app/index.php on line 25

Dopo l'aggiornamento di Joombla si riceve errore 500 quando si va in modifica del template Themler

1. Open theme folder on the server: www/Joomla_directory/templates;
2. open /app folder;
3. open index.php for editing in any html or text editor;
4. find the line
   require_once JPATH_BASE . DS . 'includes' . DS . 'toolbar.php';
   and replace it with the following line:
   $prefix = version_compare(JVERSION, '3.9', '>=') ? 'sub' : '';
require_once JPATH_BASE . DS . 'includes' . DS . $prefix . 'toolbar.php';;
5. open /app/start folder;
6. open data.php file for editing and follow step 4;
7. open manifest.php file for editing and follow step 4;
8. start Themler.

https://answers.themler.io/articles/173620/how-to-fix-the-error-after-updating-to-joomla-3-9