#vulnerable code 
                                @Test
    public void testFindOneByIdWithObjectId() {
        JacksonDBCollection<StringId, String> coll = getCollection(StringId.class, String.class);
        StringId object = new StringId();
        net.vz.mongodb.jackson.WriteResult<StringId, String> writeResult = coll.insert(object);
        assertThat(writeResult.getDbObject().get("_id"), instanceOf(org.bson.types.ObjectId.class));
        String id = writeResult.getSavedId();
        assertThat(id, instanceOf(String.class));
        StringId result = coll.findOneById(id);
        assertThat(result._id, Matchers.equalTo(id));
    } 
                            #location 10 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testFindOneByIdWithObjectId() {
        JacksonDBCollection<StringId, String> coll = getCollection(StringId.class, String.class);
        StringId object = new StringId();
        coll.insert(object);
        assertThat(coll.getDbCollection().findOne().get("_id"), instanceOf(org.bson.types.ObjectId.class));
        String id = coll.findOne()._id;
        assertThat(id, instanceOf(String.class));
        StringId result = coll.findOneById(id);
        assertThat(result._id, Matchers.equalTo(id));
    }