#vulnerable code 
                                @Test
    public void testChangedPropertyDetected() {

        ClassInfo classInfo = metaData.classInfo(Teacher.class.getName());
        Teacher teacher = new Teacher("Miss White");

        objectMemo.remember(teacher, classInfo);

        teacher.setId(115L); // the id field must not be part of the memoised property list
        teacher.setName("Mrs Jones"); // the teacher's name property has changed.

        assertFalse(objectMemo.remembered(teacher, classInfo));
    } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testChangedPropertyDetected() {
        Teacher teacher = new Teacher("Miss White");

        teacher.setId(115L); // the id field must not be part of the memoised property list
        mappingContext.remember(teacher);

        teacher.setName("Mrs Jones"); // the teacher's name property has changed.
        assertTrue(mappingContext.isDirty(teacher));
    }