Exception while axiom deletion when it reuse external components

[sszuev]

The testcase:

    String ns = "http://x#";
    OntologyManager m = OntManagers.createONT();

    OntologyModel o = m.createOntology();
    OntGraphModel g = o.asGraphModel();
    g.createOntClass(ns + "X").addSuperClass(g.createOntClass(ns + "Y"));
    g.write(System.out, "ttl");
    o.axioms().forEach(System.out::println);

    List<RemoveAxiom> changes = o.axioms().map(x -> new RemoveAxiom(o, x)).collect(Collectors.toList());
    m.applyChanges(changes);
    g.write(System.out, "ttl");
    o.axioms().forEach(System.out::println);