#vulnerable code 
                                @Test
    public void testObjectIdFieldAnnotated() throws Exception {
        ObjectIdFieldAnnotated o = new ObjectIdFieldAnnotated();
        JacksonDBCollection<ObjectIdFieldAnnotated, String> coll = createCollFor(o, String.class);
        WriteResult<ObjectIdFieldAnnotated, String> writeResult = coll.insert(o);
        assertThat(writeResult.getDbObject().get("_id"), instanceOf(org.bson.types.ObjectId.class));
        assertThat(writeResult.getSavedId(), instanceOf(String.class));
        assertThat(writeResult.getDbObject().get("id"), nullValue());
        ObjectIdFieldAnnotated result = coll.findOneById(writeResult.getSavedId());
        assertThat(result, notNullValue());
        assertThat(result.id, equalTo(writeResult.getSavedId()));
    } 
                            #location 11 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testObjectIdFieldAnnotated() throws Exception {
        ObjectIdFieldAnnotated o = new ObjectIdFieldAnnotated();
        o.id = new org.bson.types.ObjectId().toString();
        JacksonDBCollection<ObjectIdFieldAnnotated, String> coll = createCollFor(o, String.class);
        coll.insert(o);
        ObjectIdFieldAnnotated result = coll.findOneById(o.id);
        assertThat(result, notNullValue());
        assertThat(result.id, equalTo(o.id));
    }