VSCodium can't save file if it needs root privileges (Linux Flatpak)

cross-posted from: https://sh.itjust.works/post/35352374

If I open a text file that needs root privileges to modify, I get this error:

    
Failed to save 'this_file': Insufficient permissions. Select 'Retry as Sudo' to retry as superuser.


  

That's fine, that's expected.

But when I click on "Retry as Sudo", I get this error:

    
Failed to save 'this_file': Unable to find pkexec or kdesudo.

  

This happens though pkexec is installed on my machine, at /usr/bin/pkexec; and I granted VSCodium access to it through Flatseal. How do I fix this?