Improve export handling in app.py: add a conditional check to initialize the export_merge session state variable only if it doesn't already exist, ensuring data integrity during portfolio export operations.

app.py

@@ -1022,7 +1022,8 @@ with tab2:
                 )
                 st.session_state['portfolio'] = st.session_state['portfolio'][lock_mask]
             st.session_state['portfolio'] = st.session_state['portfolio'].sort_values(by='median', ascending=False)
-            st.session_state['export_merge'] = st.session_state['portfolio'].copy()
+            if 'export_merge' not in st.session_state:
+                st.session_state['export_merge'] = st.session_state['portfolio'].copy()
             with st.expander("Download options"):
                 if stack_dict is not None:
                     download_type = st.selectbox("Simple or Advanced Download?", options=['Simple', 'Advanced'], key='download_choice')