If you updated to the latest version and the following error appears when you try to launch Themler please follow the instruction below:

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

  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.