Steps to reproduce the issue

Install some extension, in my case it was Phoca Photo. After installing it and going to the extension I get this error:
Class 'JToolBarHelper' not found

Expected result

JToolbar classes will be automatically loaded

Actual result

JToolbar classes are not automatically loaded, they must be individually loaded, e.g. by:

if(!class_exists('JToolbar')) {
   require_once JPATH_SITE . '/libraries/cms/toolbar/toolbar.php';
}
if(!class_exists('JToolbarHelper')) {
   require_once JPATH_SITE . '/libraries/cms/toolbar/helper.php';
}

System information (as much as possible)

Joomla! 4, PHP 7, MySQL 5