fixing the none output error

app.py

@@ -103,6 +103,21 @@ def get_opeanai_key(openai_key):
 
 
 def slow_echo(usr_message, chat_history):
+    
+    chat_model = ChatOpenAI(temperature=0, model_name='gpt-3.5-turbo', openai_api_key=OPENAI_API_KEY)
+    # customized memory
+    memory = ConversationBufferMemory(
+          return_messages=True,
+          output_key='result'
+    )
+    
+    answer_chain = RetrievalQA.from_chain_type(
+          chat_model,
+          retriever=VectorStore.as_retriever(search_type="similarity", k=6),
+          memory = memory,
+          chain_type_kwargs={"prompt": QA_CHAIN_PROMPT},
+          return_source_documents=True
+    )
     try:
         # Attempt to get a response from the OpenAI model
         bot_result = answer_chain({"query": usr_message})