Allow to delete an event with cart positions

src/pretix/base/models/event.py

@@ -752,6 +752,7 @@ class Event(EventMixin, LoggedModel):
         return not self.orders.exists() and not self.invoices.exists()
 
     def delete_sub_objects(self):
+        self.cartposition_set.all().delete()
         self.items.all().delete()
         self.subevents.all().delete()