Fix KeyError in question_is_visible if question dependency is unknown

src/pretix/base/models/orders.py

@@ -1043,6 +1043,8 @@ class AbstractPosition(models.Model):
         }
 
         def question_is_visible(parentid, qvals):
+            if parentid not in question_cache:
+                return False
             parentq = question_cache[parentid]
             if parentq.dependency_question_id and not question_is_visible(parentq.dependency_question_id, parentq.dependency_values):
                 return False