update app.py

app.py

@@ -123,7 +123,7 @@ end_date_str = end_date.strftime('%Y-%m-%d')
 if 'results' not in st.session_state:
     st.session_state.results = []
 if 'last_params' not in st.session_state:
-    st.session_state.last_params = None
+    st.session_state.last_params = {}
 if 'map_data' not in st.session_state:
     st.session_state.map_data = None  # Initialize map_data