Edge case: index out of bounds on saving first tab

[SaraWolffs]

Haven't produced a reliable repro yet, but under some conditions, trying to save the first tab throws IndexOutOfBounds(-1,n) where n is the number of open tabs. Here's a trace for 9 tabs: exception.txt

I'll come back to this at some point, see if I can create a reliable reproduction, but maybe someone can tell from the stack trace alone what happened.