#vulnerable code 
                                public UpdateResult save(T object, WriteConcern concern) throws MongoWriteException, MongoWriteConcernException, MongoException {
        Document dbObject = convertToDocument(object);
        Object _id = dbObject.get("_id");
        if(_id == null) {
            this.insert(object, concern);
            return UpdateResult.acknowledged(0, 1L, new BsonObjectId((ObjectId) convertToDocument(object).get("_id")));
        } else {
            return this.replaceOne(new Document("_id", _id), object, true, concern);
        }
    } 
                            #location 3 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public UpdateResult save(T object, WriteConcern concern) throws MongoWriteException, MongoWriteConcernException, MongoException {
        Object _id;
        @SuppressWarnings("unchecked")
        final Codec<T> codec = getMongoCollection().getCodecRegistry().get((Class<T>) object.getClass());
        if (codec instanceof CollectibleCodec) {
            _id = JacksonCodec.extractValueEx(((CollectibleCodec<T>) codec).getDocumentId(object));
        } else {
            Document dbObject = convertToDocument(object);
            _id = dbObject.get("_id");
        }
        if(_id == null) {
            if (concern == null) {
                this.insert(object);
            } else {
                this.insert(object, concern);
            }
            if (codec instanceof CollectibleCodec) {
                return UpdateResult.acknowledged(0, 1L, ((CollectibleCodec<T>)codec).getDocumentId(object));
            } else {
                return UpdateResult.acknowledged(0, 1L, null);
            }
        } else {
            return this.replaceOne(new Document("_id", _id), object, true, concern);
        }
    }