Update export file handling in app.py: add logic to initialize the export file from the portfolio when no export file exists, enhancing data management and ensuring a seamless user experience during portfolio updates.

app.py

@@ -939,6 +939,8 @@ with tab3:
                                 if col not in excluded_cols:
                                     st.session_state['export_file'][col] = st.session_state['export_file'][col].map(st.session_state['export_dict'])
                             st.write('Export portfolio updated!')
+                else:
+                    st.session_state['export_file'] = st.session_state['portfolio'].copy()
             if 'export_file' in st.session_state:    
                 st.download_button(label="Download Portfolio", data=st.session_state['export_file'].to_csv(index=False), file_name="portfolio.csv", mime="text/csv")
             else: