Debian 10 - LCNC2.9 : Cannot edit file as root

Since I upgraded to Debian 10 (Buster) and LinuxCNC 2.9 I haven't been able to open any text editor whilst as Root (Sudo su -) via Terminal.
I can open Mousepad or Geany from the Accessory menu, but as soon as I go through Terminal I have errors refaring to GTK.
What's happened here? How are we supposed to edit files via Root ?

?
then open the file to edit and your file editor will be su authentication level

Hi,

Maybe "sudo apt-get reinstall sudo"
Or "sudo adduser user" for testing. Check : "sudo adduser root"
Or "sudo passwd" or "sudo passwd root"
If nothing works..

We can edit a systemd service.