Repository: langchain
Issue: Error in RootListenersTracer.on_chain_end callback: KeyError('answer') while streaming a RAG Chain Output
Labels: 🤖:bug,investigate,🔌: openai

### Checked other resources

- [x] I added a very descriptive title to this issue.
- [X] I searched the LangChain documentation with the integrated search.
- [X] I used the GitHub search to find a similar question and didn't find it.
- [X] I am sure that this is a bug in LangChain rather than my code.
- [X] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

### Example Code

I have the following code for building a RAG Chatbot (using [this](https://python.langchain.com/v0.2/docs/how_to/streaming/) example):  

```
from langchain_openai import OpenAIEmbeddings, ChatOpenAI
from langchain.chains import create_retrieval_chain
from langchain.chains.history_aware_retriever import create_history_aware_retriever
from langchain.chains.combine_documents import create_stuff_documents_chain

vectordb = FAISS.load_local(persist_directory, embedding, index_name, allow_dangerous_deserialization=True)
retriever=vectordb.as_retriever()

llm = ChatOpenAI()

....
prompt={.....}
....

question_answer_chain = create_stuff_documents_chain(llm, prompt, output_parser=parser)
rag_chain = create_retrieval_chain(history_aware_retriever, question_answer_chain)

conversational_rag_chain = RunnableWithMessageHistory(
    rag_chain,
    get_session_history,
    input_messages_key="input",
    history_messages_key="chat_history",
    output_messages_key="answer",
)

while True:
    query = input("Ask a question: ")

    for chunk in conversational_rag_chain.stream(
        {"input": query,},
        config={
            "configurable": {
                "session_id": "demo_1"
            }
        }
    ):
        if answer_chunk := chunk.get("answer"):
            print(f"{answer_chunk}", end="", flush=True)
    
    print()
```

### Error Message and Stack Trace (if applicable)

```
Ask a question: How many colors are in rainbow?
Error in RootListenersTracer.on_chain_end callback: KeyError('answer')
Error in callback coroutine: KeyError('answer')
A rainbow typically has seven colors, which are: Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Ask a question: 
```

### Description

Hi,

I am trying to get the answer as `stream`, the problem is whenever the `conversational_rag_chain.stream()` is initiating, using an `input` it is giving the following errors:  
`Error in RootListenersTracer.on_chain_end callback: KeyError('answer')`  
`Error in callback coroutine: KeyError('answer')`  
and then the output is printing as intended.

My question is, how can I solve it? I have entered `output_messages_key="answer"` in the `conversational_rag_chain` already, so am I doing something wrong or is a `bug`?

Any little discussion or help is welcome. Thanks in advance.

### System Info

System Information
------------------
> OS:  Linux
> OS Version:  #39-Ubuntu SMP PREEMPT_DYNAMIC Fri Jul  5 21:49:14 UTC 2024
> Python Version:  3.12.3 (main, Apr 10 2024, 05:33:47) [GCC 13.2.0]

Package Information
-------------------
> langchain_core: 0.2.23
> langchain: 0.2.11
> langchain_community: 0.2.10
> langsmith: 0.1.93
> langchain_chroma: 0.1.2
> langchain_openai: 0.1.17
> langchain_text_splitters: 0.2.2

Packages not installed (Not Necessarily a Problem)
--------------------------------------------------
The following packages were not found:

> langgraph
> langserve