At the moment, we don't have any guards in the targetList to _not_ execute the
creation/destruction listeners once the toolbox gets destroyed.
We only have a stopListening function on the targetList that is called when we
close the toolbox, but we can't rely only on that since it's also called when
doing a target switch (and working around that is very racy).

One solution would be to follow the common pattern we have everywhere by having
a destroy method that we would check before trying to call the listeners callback.

This might help with intermittent test failures.

Differential Revision: https://phabricator.services.mozilla.com/D88765