#fixed code
                                @Test
    public void testDefaultConfig() {
        Subject subject = sm.getSubject();

        AuthenticationToken token = new UsernamePasswordToken("guest", "guest");
        subject.login(token);
        assertTrue(subject.isAuthenticated());
        assertTrue("guest".equals(subject.getPrincipal()));
        assertTrue(subject.hasRole("guest"));

        Session session = subject.getSession();
        session.setAttribute("key", "value");
        assertEquals(session.getAttribute("key"), "value");

        subject.logout();

        assertNull(subject.getSession(false));
        assertNull(subject.getPrincipal());
        assertNull(subject.getPrincipals());
    }
#vulnerable code 
                                @Test
    public void testDefaultConfig() {
        Subject subject = SecurityUtils.getSubject();

        AuthenticationToken token = new UsernamePasswordToken("guest", "guest");
        subject.login(token);
        assertTrue(subject.isAuthenticated());
        assertTrue("guest".equals(subject.getPrincipal()));
        assertTrue(subject.hasRole("guest"));

        Session session = subject.getSession();
        session.setAttribute("key", "value");
        assertEquals(session.getAttribute("key"), "value");

        subject.logout();

        assertNull(subject.getSession(false));
        assertNull(subject.getPrincipal());
        assertNull(subject.getPrincipals());
    } 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE