Enhance custom export functionality in app.py: add a check for the presence of 'export_merge' before allowing row selection, improving robustness and preventing errors during user interaction.

app.py

@@ -1210,9 +1210,10 @@ 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:
-                    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]])
+                    if st.session_state['export_merge'] is not None:
+                        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]])
                         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 clear_export: