* 겪고 있는 문제 상황을 최대한 자세하게 작성해주세요.

* 문제 해결을 위해 어떤 시도를 해보았는지 구체적으로 함께 알려주세요.

끝! 버튼을 누르면 화면 하단에 오류가 떠요

작성한 코드 및 에러 메세지

//gameManager.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using System.Linq;


public class gameManager : MonoBehaviour
{
    public Text timeTxt;
    public GameObject endTxt;
    public GameObject card;
    float time;
    public static gameManager I;
    public GameObject firstCard;
    public GameObject secondCard;


    void Awake()
    {
        I = this;
    }


    // Start is called before the first frame update
    void Start()
    {
        int[] rtans = { 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7 };
        rtans = rtans.OrderBy(item => Random.Range(-1.0f, 1.0f)).ToArray();
        for (int i = 0; i < 16; i++)
        {
            GameObject newCard = Instantiate(card);
            newCard.transform.parent = GameObject.Find("cards").transform;


            float x = (i / 4) * 1.4f - 2.1f;
            float y = (i % 4) * 1.4f - 3.0f;
            newCard.transform.position = new Vector3(x, y, 0);


            string rtanName = "rtan" + rtans[i].ToString();
            newCard.transform.Find("front").GetComponent<SpriteRenderer>().sprite = Resources.Load<Sprite>(rtanName);


        }
    }


    // Update is called once per frame
    void Update()
    {
        time += Time.deltaTime;
        timeTxt.text = time.ToString("N2");


    }
    public void isMatched()
    {
        string firstCardImage = firstCard.transform.Find("front").GetComponent<SpriteRenderer>().sprite.name;
        string secondCardImage = secondCard.transform.Find("front").GetComponent<SpriteRenderer>().sprite.name;


        if (firstCardImage == secondCardImage)
        {
            firstCard.GetComponent<card>().destroyCard();
            secondCard.GetComponent<card>().destroyCard();


            int cardsLeft = GameObject.Find("cards").transform.childCount;
            if (cardsLeft == 2)
            {
                Time.timeScale = 0f;
                endTxt.SetActive(true);
                Invoke("GameEnd", 1f);
            }


        }
        else
        {
            firstCard.GetComponent<card>().closeCard();
            secondCard.GetComponent<card>().closeCard();


        }


        firstCard = null;
        secondCard = null;


    }
    void GameEnd()
    {
        Time.timeScale = 0f;
        endTxt.SetActive(true);
    }
}

//endtxt.cs

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


public class endTxt : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        
    }


    // Update is called once per frame
    void Update()
    {
        
    }


    public void ReGame()
    {
        SceneManager.LoadScene("MainScene");
    }
}