add_member fails after new install with ALLOW_REGISTRATION=FALSE

After a new Ubuntu server install:
While ALLOW_REGISTRATION = False
When trying to add a member to a project...

add_member fails indicating debug message:

Request Method: | POST
-- | --
http://editor4.vmath.club/project/2/settings/add_member
2.2.10
NoReverseMatch
Reverse for 'registration_register' not found. 'registration_register' is not a valid view function or pattern name.
/opt/python/numbas-editor/lib/python3.6/site-packages/django/urls/resolvers.py in _reverse_with_prefix, line 673
/opt/python/numbas-editor/bin/python
3.6.9
['/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', '/opt/python/numbas-editor/lib/python3.6/site-packages', '/srv/www/numbas_editor', '/srv/numbas/compiler/bin']

Start by tracing the POST handler for /project/2/settings/add_member and the reference to the missing registration_register URL in the Django traceback. Reproduce the failure on a fresh install with ALLOW_REGISTRATION=False; done means adding a project member succeeds without the NoReverseMatch error.

Written by the indexing model from the issue text.