This is because tabgroups are being removed. See https://support.mozilla.org/en-US/kb/tab-groups-removal
We probably should prep for that. In the meantime, if you install @Quicksaver's tabgroup plugin, it'll keep it working and keep tab groups around as well!

FWIW I was going through all instances of tabGroup.tabView thinking an easy fix would be to null-check them all. But quite a lot of other things depend on it, and I'm not familiar with a lot of them. I'm sorry I can't write a follow-up to my previous tabview patch, but I don't feel I'm the most adequate person for that. :(

@Quicksaver you're probably just a bit less familiar with the codebase than I am and you're definitely more familiar with firefox plugin internals than I am so I don't think you're doing a bad job :P
Appreciate any help you can provide.

thinking an easy fix would be to null-check them all. But quite a lot of other things depend on it

Took another look, I definitely misinterpreted a lot of it the first time. Anyway, PR #414. :)

Thanks, I installed that tabgroup plugin and at least now the annoying message is gone again :)

Should be fixed now as part of #414