AttributeError with CreateMembership on a worker without a repository that you do not have access to

Sentry Issue: ARKINDEX-BACKEND-1TY

AttributeError: 'NoneType' object has no attribute 'memberships'
(11 additional frame(s) were not displayed)
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  File "arkindex/users/serializers.py", line 371, in validate
  File "arkindex/project/mixins.py", line 102, in get_max_level
  File "arkindex/users/utils.py", line 101, in get_max_level

The deprecated WorkerACLMixin has a get_max_level method, different from arkindex.users.utils.get_max_level, which is only used by CreateMembership and does not support workers without repositories.