Although there's no forum yet for the Theta Platform, I have a new 2020 Terrain and tonight I got the same "blue screen" of death regarding "service the rear vision system". I went to move the vehicle and started it and quickly shifted into reverse and the issue showed up.
Had the vehicle for 1 week. Miss my Buick LaCrosse already. Thanks for the comments here. I'll let you know how they handle this in Texas. There is a service bulletin (19-NA-076) on this problem from April 2019 so this has been an issue for some time for GM.
https://static.nhtsa.gov/odi/tsbs/2019/MC-10160418-9999.pdf
Checked the vehicle this morning and the message is gone and the cameras seem to work now. Ugh...