To reproduce, just delete the first line in a version-controlled document. I believe the reason is that changebar markers of deleted lines are always placed on a previous line and there's no previous line in this case.

This also affects #531 Because in this case it isn't possible to perform undo. Note that if this issue is fixed somehow, the code from #531 has to be updated too because right now it assumes the deleted line's marker is on the previous line.