The buttons don't check whether the operation they correspond to is
possible to perform in the current state of the selection box, so not
checking `Can{Reverse,Rotate}` causes superfluous undo states to be
added without any real changes if an attempt is made to reverse or
rotate a selection that cannot be reversed or rotated.
In this particular case guarding with `.IsNull()` makes no sense, as the
`Trigger` is supplied in constructor and cannot feasibly be null. Doing
that only makes sense in scenarios where BDL / dependency injection is
involved.