fix retriever function

agent.py

@@ -212,11 +212,19 @@ def build_graph(provider: str = "groq"):
     def retriever(state: MessagesState):
         """Retriever node"""
         similar_question = vector_store.similarity_search(state["messages"][0].content)
-        example_msg = HumanMessage(
-            content=f"Here I provide a similar question and answer for reference: \n\n{similar_question[0].page_content}",
-        )
+        
+        if not similar_question:
+            example_msg = HumanMessage(
+                content="I couldn't find any similar question or answer in the database."
+            )
+        else:
+            example_msg = HumanMessage(
+                content=f"Here I provide a similar question and answer for reference: \n\n{similar_question[0].page_content}"
+            )
+        
         return {"messages": [sys_msg] + state["messages"] + [example_msg]}
 
+
     builder = StateGraph(MessagesState)
     builder.add_node("retriever", retriever)
     builder.add_node("assistant", assistant)