Fix instance selection mode destruction (#7447)
Fixes #7447 (closed).
The stack trace pointed out that an uninitialized base node might be dereferenced (e.g. if you never selected anything). This is the kind of check that other instances of base node removal already do. To be honest, I have zero idea why this destructor wouldn't cause it in other conditions, but this does fix the crash seemingly without any repercussions, so I can't complain.