Update handler.py

handler.py

@@ -20,7 +20,7 @@ from langchain.chains.combine_documents import create_stuff_documents_chain
 from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder
 from langchain_core.messages import HumanMessage
 from langchain_core.output_parsers import StrOutputParser
-from langchain_core.runnables import RunnableBranch
+from langchain_core.runnables import RunnableBranch, RunnablePassthrough
 from operator import itemgetter
 
 from langchain.memory import ConversationBufferMemory