Do not crash when trying to determine author roles in discussion topics

owl/post.py

@@ -208,7 +208,7 @@ def topic_post(form, student_uid) -> Optional[db.Post]:
     if comment or attach or points is not None:
         if g.is_teacher:
             role = db.PostRole.teacher
-        elif g.uid == student_uid:
+        elif g.uid == student_uid or g.topic.type == 'D':
             role = db.PostRole.student
         elif g.is_grader:
             role = db.PostRole.grader