Update app.py

app.py

@@ -163,6 +163,9 @@ graph = builder.compile()
 st.set_page_config(page_title="Math Chatbot",layout="centered")
 st.title("Math Chatbot")
 
+if "chat_history" not in st.session_state:
+    st.session_state.chat_history = []
+    
 for msg in st.session_state.chat_history:
     with st.chat_message(msg["role"]):
         st.markdown(msg["content"])