error message

app.py

@@ -37,6 +37,8 @@ def initialize_index(index_name, documents_folder):
 
 @st.cache_data(max_entries=200, persist=True)
 def query_index(_index, query_text):
+    if _index is None:
+        return "Please initialize the index!"
     response = _index.as_query_engine().query(query_text)
     return str(response)