Update download button logic in app.py: ensure the download button for the portfolio is only displayed when the export file is available, improving user experience and preventing errors during download attempts.

app.py

@@ -950,8 +950,8 @@ with tab3:
                                     export_file[col] = export_file[col].map(st.session_state['export_dict'])
                             st.write('Export portfolio updated!')
                             st.table(export_file)
-                
-            st.download_button(label="Download Portfolio", data=export_file.to_csv(index=False), file_name="portfolio.csv", mime="text/csv")
+            if export_file is not None:    
+                st.download_button(label="Download Portfolio", data=export_file.to_csv(index=False), file_name="portfolio.csv", mime="text/csv")
             
             # Add pagination controls below the dataframe
             total_rows = len(st.session_state['portfolio'])