#fixed code
                                @Test
    public void testObjectIdAnnotationOnStringSaved() {
        StringId object = new StringId();
        String id = new org.bson.types.ObjectId().toString();
        object._id = id;

        JacksonDBCollection<StringId, String> coll = getCollection(StringId.class, String.class);

        coll.insert(object);
        StringId result = coll.findOneById(id);
        assertThat(result._id, equalTo(id));
        assertThat(coll.getDbCollection().findOne().get("_id").toString(), equalTo(id));
    }
#vulnerable code 
                                @Test
    public void testObjectIdAnnotationOnStringSaved() {
        StringId object = new StringId();
        String id = new org.bson.types.ObjectId().toString();
        object._id = id;

        JacksonDBCollection<StringId, String> coll = getCollection(StringId.class, String.class);

        coll.insert(object);
        StringId result = coll.findOneById(id);
        assertThat(result._id, equalTo(id));
    } 
                            #location 11 
                            #vulnerability type NULL_DEREFERENCE