Cannot save events with participants

Thalia Technicie
2019-12-13T15:41:54+01:00

Sentry Issue: [CONCREXIT-1P](https://sentry.io/organizations/thalia/issues/1375339372/?referrer=gitlab_integration)

```
AssertionError: Cannot filter a query once a slice has been taken.
(11 additional frame(s) were not displayed)
...
File "django/contrib/admin/options.py", line 1561, in _changeform_view
self.save_model(request, new_object, form, not add)
File "django/contrib/admin/options.py", line 1088, in save_model
obj.save()
File "events/models.py", line 398, in save
self.participants.filter(member__isnull=False)
File "django/db/models/query.py", line 892, in filter
return self._filter_or_exclude(False, *args, **kwargs)
File "django/db/models/query.py", line 904, in _filter_or_exclude
"Cannot filter a query once a slice has been taken."
```