Solved! Go to the solution
Best solution: The quick fix is to publish the theme before you installed this app.
If you didn't back up the theme before installing the app then this is a good learning experience to backup your theme before making changes 🙂 check
If you don't have a backup, check the theme.liquid file to see if you can revert to an older version (it will be a dropdown at the top of the theme editor once you open that file).
Fixing those might solve it (you should fix console errors either way).
If still not working, you can contact the app and ask them what files they modify, then revert those files individually to the state before you installed the app.