#vulnerable code 
                                @Test
    public void testObjectIdGenerated() {
        ObjectIdId object = new ObjectIdId();

        JacksonDBCollection<ObjectIdId, org.bson.types.ObjectId> coll = getCollection(ObjectIdId.class,
                org.bson.types.ObjectId.class);

        org.bson.types.ObjectId id = coll.insert(object).getSavedId();
        ObjectIdId result = coll.findOneById(id);
        assertThat(result._id, equalTo(id));
    } 
                            #location 10 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testObjectIdGenerated() {
        ObjectIdId object = new ObjectIdId();

        JacksonDBCollection<ObjectIdId, org.bson.types.ObjectId> coll = getCollection(ObjectIdId.class,
                org.bson.types.ObjectId.class);

        coll.insert(object);
        org.bson.types.ObjectId id = coll.findOne()._id;
        ObjectIdId result = coll.findOneById(id);
        assertThat(result._id, equalTo(id));
        assertThat((org.bson.types.ObjectId) coll.getDbCollection().findOne().get("_id"), equalTo(id));
    }