I can confirm that the same issue happened with Sitemap by BestWebSoft
BestWebSoft made an FAQ for this issue.
https://support.bestwebsoft.com/hc/en-us/articles/19682480093981-I-have-problems-after-opening-the-WordPress-admin-panel
You don’t need to FTP to edit the functions.php. Go to [your-domain]/wp-admin/theme-editor.php
Edit: Didnt notice there was a second page….
