Prevent registering for event without time places

Also prevent admins, because if a standalone event were to be changed to repeating, there would be no time places to transfer the tickets to
(after having implemented the changes mentioned in #563).

news/models.py

@@ -149,7 +149,11 @@ def standalone(self):
         return self.event_type == self.Type.STANDALONE
 
     def can_register(self, user: User, *, fail_if_not_standalone):
-        # Admins should always be allowed
+        # Registering for an event with no time places should never be allowed - no matter the `event_type`
+        if not self.timeplaces.exists():
+            return False
+
+        # Admins should always be allowed (except for the above case)
         if user.has_perm('news.cancel_ticket'):
             return True
 

news/views.py

@@ -518,6 +518,10 @@ def setup(self, request, *args, **kwargs):
         time_place_pk = self.kwargs.get('time_place_pk')
         if time_place_pk is None:
             self.ticket_time_place = None
+            # Raise an error if the event has no time places (see the first `timeplaces` check in `Event.can_register()`)
+            if not self.event.timeplaces.exists():
+                raise Http404("Cannot register for an event with no time places")
+
             self.ticket_event = self.event
         else:
             self.ticket_time_place = get_object_or_404(TimePlace, pk=time_place_pk, event=self.event)