UnityのCscriptでエラー

前提・実現したいこと

unity で矢が落ちてきてプレイヤーに当たるとhpゲージが減るscriptでエラーが出ました。

発生している問題・エラーメッセージ

エラーメッセージ
NullReferenceException: Object reference not set to an instance of an object
ArrowController.Update () (at Assets/ArrowController.cs:32)

該当のソースコード

ソースコード
GameDirector.cs
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class GameDirector : MonoBehaviour
{
    GameObject hpGauge;
    // Start is called before the first frame update
    void Start()
    {
        this.hpGauge = GameObject.Find("hpGauge");
    }

    // Update is called once per frame
    public void DecreaseHp(){
        this.hpGauge.GetComponent<Image>().fillAmount -= 0.1f;
    }
}



ArrowController.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ArrowController : MonoBehaviour
{
    GameObject player;
    // Start is called before the first frame update
    void Start()
    {
        this.player = GameObject.Find("player");
    }

    // Update is called once per frame
    void Update()
    {
        transform.Translate(0,-0.1f,0);

        if(transform.position.y < -5.0f){
            Destroy(gameObject);
        }

        Vector2 p1 = transform.position;
        Vector2 p2 = this.player.transform.position;
        Vector2 dir = p1 - p2;
        float d = dir.magnitude;
        float r1 = 0.5f;
        float r2 = 1.0f;

        if(d < r1 + r2){
            GameObject director = GameObject.Find("GameDirector");
            director.GetComponent<GameDirector>().DecreaseHp();

            Destroy(gameObject);
        }

    }
}