dispose method
Called by deactivate; should contain the object deactivation code.
If propose throws an Object, the default Actor implementation will
call dispose to clean up even a partially initialized object. The
deactivate method passes its argument to this method as a thrown
argument. A non-null thrown indicates that dispose was called during
the handling of a thrown object.
Implementation
@protected
@override
void dispose() {
_forwardFirst?.close();
_forwardFirst = null;
_forwardNext?.close();
_forwardNext = null;
_reverseFirst?.close();
_reverseFirst = null;
_reverseNext?.close();
_reverseNext = null;
super.dispose();
}