Bug: 500 server error when searching for an organization in admin

[HaraldWalker]

Describe the bug When searching for a organization in the admin interface using the search field it throws a 500 server error.

To Reproduce Steps to reproduce the behavior:

1. Go to Django admin.
2. Go to organisations.
3. Enter a search string and click on search.
4. See error

Expected behavior It shows the organisations that match the search.

Screenshots Error message: Cannot resolve keyword 'url-slug' into field.

Context Seen on production, also reproducible locally.