,MissingReferenceException: The object of type 'Transform' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object.

I have problem that my player is not spawning MissingReferenceException: The object of type 'Transform' has been destroyed but you are still trying to access it.

{ public static GameMaster gm;

 [SerializeField]
 private int maxLives = 3;
 private static int _remainingLives;
 public static int RemainingLives
 {
     get { return _remainingLives; }
 }

 [SerializeField]
 private int startingMoney;
 public static int Money;

 void Awake()
 {
     if (gm == null)
     {
         gm = GameObject.FindGameObjectWithTag("GM").GetComponent<GameMaster>();
     }
 }

 public Transform playerPrefab;
 public Transform spawnPoint;
 public float spawnDelay = 2;
 public Transform spawnPrefab;
 public string respawnCountdownSoundName = "RespawnCountdown";
 public string spawnSoundName = "Spawn";

 public string gameOverSoundName = "GameOver";

 public CameraShake cameraShake;

 [SerializeField]
 private GameObject gameOverUI;

 [SerializeField]
 private GameObject upgradeMenu;

 public delegate void UpgradeMenuCallback(bool active);
 public UpgradeMenuCallback onToggleUpgradeMenu;

 //cache
 private AudioManager audioManager;
 private UnityEngine.Object clone;
 private object _enemy;

 void Start()
 {
     if (cameraShake == null)
     {
         Debug.LogError("No camera shake referenced in GameMaster");
     }

     _remainingLives = maxLives;

     Money = startingMoney;

     //caching
     audioManager = AudioManager.instance;
     if (audioManager == null)
     {
         Debug.LogError("FREAK OUT! No AudioManager found in the scene.");
     }
 }

 void Update()
 {
     if (Input.GetKeyDown(KeyCode.U))
     {
         ToggleUpgradeMenu();
     }
 }

 private void ToggleUpgradeMenu()
 {
     upgradeMenu.SetActive(!upgradeMenu.activeSelf);
     onToggleUpgradeMenu.Invoke(upgradeMenu.activeSelf);
 }

 public void EndGame()
 {
     audioManager.PlaySound(gameOverSoundName);

     Debug.Log("GAME OVER");
     gameOverUI.SetActive(true);
 }

 public IEnumerator _RespawnPlayer()
 {
     yield return new WaitForSeconds(spawnDelay);

     Instantiate(playerPrefab, spawnPoint.position, spawnPoint.rotation);
     Transform clone = (Transform)Instantiate(spawnPrefab, spawnPoint.position, spawnPoint.rotation);
 }

 public static void KillPlayer(Player player)
 {
     Destroy(player.gameObject);
     _remainingLives -= 1;
     if (_remainingLives <= 0)
     {
         gm.EndGame();
     }
     else
     {
         gm.StartCoroutine(gm._RespawnPlayer());
     }
 }

 public static void KillEnemy(Enemy enemy)
 {
     gm._KillEnemy(enemy);
 }
 public void _KillEnemy(Enemy _enemy)
 {
     // Let's play some sound
     audioManager.PlaySound(_enemy.deathSoundName);

     // Add particles
     GameObject _clone = Instantiate(_enemy.deathParticles, _enemy.transform.position, Quaternion.identity) as GameObject;
     Destroy(_clone, 5f);
 }

 private class AudioManager
 {
     internal static readonly AudioManager instance;

     internal void PlaySound(string gameOverSoundName)
     {
         throw new NotImplementedException();
     }
 }

} ,