Error when trying to add contact to protected group from webform

[ErikHommel]

At PUM Senior Experts (funding this extension) they want to protect the adding/removing of group members from within CiviCRM (from the UI) but it is allowed if an anonymous user applies as candidate expert with a webform.
The extension now stops halfway the total process of applying as an expert in the CiviCRM part because the check fails and redirects to an empty page.

[ErikHommel]

Solution is to add a test if the request is from a Drupal webform, and skip test if so.