Custom fields can be deleted after use

Version 16.8.36 supposedly prevented deletion of custom fields already in use. That protection seems to have been lost at some point. At least by version 17.11.21, custom fields that had content on completed forms could be deleted without even a warning. The problem exists for custom fields for both customers and suppliers, and may exist for other categories, as well.

Fixed in the latest version (17.11.24)