Выдаёт ошибку в коде.Помогите

(выдаёт ошибку):NullReferenceException: Object reference not set to an instance of an object
MainMenu.Update () (at Assets/Skripts/Menu.cs:51) (ошибка на 51 строке)

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.SceneManagement;
public class MainMenu : MonoBehaviour
{
[SerializeField] int money;
public int total_money;
public Text moneyText;

private void Start()
{
money = PlayerPrefs.GetInt("money");
total_money = PlayerPrefs.GetInt("total_money");
bool isFirst = PlayerPrefs.GetInt("isFirst") == 1 ? true : false;
if (isFirst)
{
StartCoroutine (IdleFarm());
}
}

public void ButtonClick()
{
money++;
total_money++;
PlayerPrefs.SetInt("money", money);
PlayerPrefs.SetInt("total_money", total_money);
}

IEnumerator IdleFarm()
{
yield return new WaitForSeconds(1);
money++;
Debug.Log(money);
PlayerPrefs.SetInt("money", money);
StartCoroutine(IdleFarm());
}


public void ToAchievements()
{
SceneManager.LoadScene(1);
}



// Update is called once per frame
void Update()
{
moneyText.text = money.ToString();
}
}