Fixed a crash on deleting an object from the platter due to the object_list_changed() function calling GLVolumeCollection::check_outside_state() on GLVolumes, which were already deleted.
Fixed a crash on deleting an object from the platter due to the
object_list_changed() function calling GLVolumeCollection::check_outside_state()
on GLVolumes, which were already deleted.

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r