Fix error message for image access. (#3936)

### What problem does this PR solve?

#3883

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/services/conversation_service.py

@@ -47,6 +47,8 @@ class ConversationService(CommonService):
 
 def structure_answer(conv, ans, message_id, session_id):
     reference = ans["reference"]
+    if not isinstance(reference, dict):
+        reference = {}
     temp_reference = deepcopy(ans["reference"])
     if not conv.reference:
         conv.reference.append(temp_reference)