@theslik It is basically GNOME, but it is the modified version used by Pop OS. (I think it might be called COSMIC — not the new COSMIC being worked on by System76.)
@theslik If you're curious, I would get the X-shaped X Window System cursor that means you're pointing at the root window (or something) when I would point away from the error dialog. And when I clicked through, GNOME/COSMIC started up.