Merge branch 'fix/604' into 'develop'

Fix public_speakers

Closes #604

See merge request hub/hub!1036

src/core/models/events.py

@@ -182,7 +182,7 @@ class Event(TaggedItemMixin, BackendMixin, ActivityLogMixin, models.Model):
         persons = [participant.participant for participant in self.participants.filter(is_public=True, role=EventParticipant.Role.SPEAKER)]
 
         if len(persons) == 0 and self.kind == Event.Kind.SELF_ORGANIZED and self.owner:
-            persons.add(self.owner)
+            persons.append(self.owner)
 
         return persons