debuging none output issue

app.py

@@ -76,13 +76,13 @@ def slow_echo(usr_message, chat_history):
           else:
             source_print[source_doc[i]] = 'page: '+ source_page[i]
 
-        bot_response = bot_response + '\n Source:'
+        bot_response += '\n Source:'
         for doc, page in source_print.items():
             bot_response += '\n' + doc + ': ' + page
 
-        time.sleep(1)
-
         chat_history.append((usr_message, bot_response))
+        
+        time.sleep(1)
         yield "", chat_history
 
     except openai.error.OpenAIError as e: