What steps will reproduce the problem?
(1) Create a group
(2) Trigger a delayed process that will delete the group (e.g. write your own extension that will close a tab after a delay)
(3) Right click on another tab and open the add to existing group submenu
(4) Wait for the delayed process to delete the group
(5) Click on the group

What is the expected result?
Nothing happens, or the menu closes when the strip is modified, or the group is resurrected with this one tab as its only member.

What happens instead?
A DCHECK or the tab is added to a different group (depending on whether there are other groups created after the deleted one).

N.B. groups aren't actually deleted yet, so for now adding to an emptied group should add to the correct group but DCHECK because it can't determine the index to insert at.