Improve UndoGroupHelper and use it more

Improve UndoGroupHelper and use it more

Now the helper class contains logic that checks whether buffer are
known before closing them. This avoids potential crashes.

Use it in different places to siplify code. It is not clear at this
point whether it should be used everywhere.

Followup to bug #10847.

(cherry picked from commit cd9e42dc3529980257d2b0fe6fd623fd2b99a1e6)