fix async error

app.py

@@ -215,10 +215,12 @@ async def main(message: cl.Message):
 
         msg = cl.Message(content="")
 
-        async for chunk in await cl.make_async(lcel_rag_chain.stream)(
+        chunks = await cl.make_async(lcel_rag_chain.stream)(
             {"query": message.content},
             config=RunnableConfig(callbacks=[cl.LangchainCallbackHandler()]),
-        ):
+        )
+        
+        async for chunk in chunks:
             await msg.stream_token(chunk)
 
         await msg.send()