AttributeError: 'NoneType' object has no attribute 'is_authenticated'

Sentry Issue: CONCREXIT-2A

AttributeError: 'NoneType' object has no attribute 'is_authenticated'
(14 additional frame(s) were not displayed)
...
  File "rest_framework/serializers.py", line 529, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "rest_framework/fields.py", line 1905, in to_representation
    return method(value)
  File "events/api/serializers.py", line 86, in _class_names
    if services.is_user_registered(self.context["member"], instance):
  File "events/services.py", line 24, in is_user_registered
    if not event.registration_required or not member.is_authenticated:
  File "django/utils/functional.py", line 225, in inner
    return func(self._wrapped, *args)