Refine custom export logic in app.py: update the condition to check for 'export_merge' in session state, enhancing error handling and ensuring smoother user interactions when selecting rows for custom export.

app.py

@@ -1210,7 +1210,7 @@ with tab2:
                         st.session_state['export_base'] = st.session_state['export_base'].drop_duplicates()
                         st.session_state['export_base'] = st.session_state['export_base'].reset_index(drop=True)
                 with partial_col:
-                    if st.session_state['export_merge'] is not None:
+                    if 'export_merge' in st.session_state:
                         select_custom_index = st.multiselect("Select rows to add to Custom Export", options=st.session_state['export_merge'].index, default=[])
                         if st.button("Add selected to Custom Export"):
                             st.session_state['export_base'] = pd.concat([st.session_state['export_base'], st.session_state['export_merge'].iloc[select_custom_index]])