Unable to delete object from a list using foreach loop

I have been unable to delete the spike Gameobject. It always throws a null reference exception error. I can't seem to figure out the issue here.
Check KillOldSpikes() the last function in the code.

(Line 225)

Code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class GameController : MonoBehaviour
 {
     public static GameController instance = null;
 
     const float width = 3.7f;
     const float height = 7f;
 
     public float snakeSpeed = 1;
 
     public BodyPart bodyPrefab = null;
     public GameObject rockPrefab = null;
     public GameObject eggPrefab = null;
     public GameObject goldEggPrefab = null;
     public GameObject spikePrefab = null;
 
     public Sprite tailSprite = null;
     public Sprite bodySprite = null;
 
     public SnakeHead snakeHead = null;
 
     public bool alive = true;
 
     public bool waitingToPlay = true;
 
     List<Egg> eggs = new List<Egg>();
     List<Egg> spikes = new List<Egg>();
 
     int level = 0;
     int noOfEggsForNextLevel = 0;
 
     public int score = 0;
     public int $$anonymous$$Score = 0;
 
     public Text scoreText = null;
     public Text $$anonymous$$ScoreText = null;
 
     public Text tapToPlayText = null;
     public Text gameOverText = null;
 
     // Start is called before the first frame update
     void Start()
     {
         instance = t$$anonymous$$s;
         Debug.Log("Starting Snake Game");
         CreateWalls();
         alive = false;
     }
 
     // Update is called once per frame
     void Update()
     {
         if (waitingToPlay)
         {
             foreach (Touch touch in Input.touches)
             {
                 if (touch.phase == TouchPhase.Ended)
                 {
                     StartGamePlay();
                 }
             }
 
             if (Input.GetMouseButtonUp(0))
                 StartGamePlay();
         }
     }
 
     public void GameOver()
     {
         alive = false;
         waitingToPlay = true;
 
         gameOverText.gameObject.SetActive(true);
         tapToPlayText.gameObject.SetActive(true);
     }
 
     void StartGamePlay()
     {
         score = 0;
         level = 0;
 
         scoreText.text = "Score = " + score;
         $$anonymous$$ScoreText.text = "HiScore = " + $$anonymous$$Score;
 
         gameOverText.gameObject.SetActive(false);
         tapToPlayText.gameObject.SetActive(false);
 
         waitingToPlay = false;
         alive = true;
 
         KillOldEggs();
         KillOldSpikes();
 
         LevelUp();
     }
 
     void LevelUp()
     {
         level++;
 
         noOfEggsForNextLevel = 4 + (level * 2);
 
         snakeSpeed = 1f + (level / 4f);
         if (snakeSpeed > 6) snakeSpeed = 6;
 
         snakeHead.ResetSnake();
         CreateEgg();
 
         //KillOldSpikes();
         CreateSpike();
 
     }
 
     public void EggEaten(Egg egg)
     {
         score++;
 
         noOfEggsForNextLevel--;
         if (noOfEggsForNextLevel == 0)
         {
             score += 10;
             LevelUp();
         }
         else if (noOfEggsForNextLevel == 1) // last egg
             CreateEgg(true);
         else
             CreateEgg(false);
 
         if (score > $$anonymous$$Score)
         {
             $$anonymous$$Score = score;
             $$anonymous$$ScoreText.text = "HiScore = " + $$anonymous$$Score;
         }
 
         scoreText.text = "Score = " + score;
 
         eggs.Remove(egg);
         Destroy(egg.gameObject);
     }
 
     void CreateWalls()
     {
         float z = -1f;
 
         Vector3 start = new Vector3(-width, -height, z);
         Vector3 finish = new Vector3(-width, +height, z);
         CreateWall(start, finish);
 
         start = new Vector3(+width, -height, z);
         finish = new Vector3(+width, +height, z);
         CreateWall(start, finish);
 
         start = new Vector3(-width, -height, z);
         finish = new Vector3(+width, -height, z);
         CreateWall(start, finish);
 
         start = new Vector3(-width, +height, z);
         finish = new Vector3(+width, +height, z);
         CreateWall(start, finish);
     }
 
     void CreateWall(Vector3 start, Vector3 finish)
     {
         float distance = Vector3.Distance(start, finish);
         int noOfRocks = (int)(distance * 3f);
         Vector3 delta = (finish - start) / noOfRocks;
 
         Vector3 position = start;
         for (int rock = 0; rock <= noOfRocks; rock++)
         {
             float rotation = Random.Range(0, 360f);
             float scale = Random.Range(1.5f, 2f);
             CreateRock(position, scale, rotation);
             position = position + delta;
         }
     }
 
     void CreateRock(Vector3 position, float scale, float rotation)
     {
         GameObject rock = Instantiate(rockPrefab, position, Quaternion.Euler(0, 0, rotation));
         rock.transform.localScale = new Vector3(scale, scale, 1);
     }
 
     void CreateEgg(bool golden = false)
     {
         Vector3 position;
         position.x = -width + Random.Range(1f, (width * 2) - 2f);
         position.y = -height + Random.Range(1f, (height * 2) - 2f);
         position.z = -1f;
         Egg egg = null;
         if (golden)
             egg = Instantiate(goldEggPrefab, position, Quaternion.identity).GetComponent<Egg>();
         else
             egg = Instantiate(eggPrefab, position, Quaternion.identity).GetComponent<Egg>();
 
         eggs.Add(egg);
     }
 
 
     void CreateSpike()
     {
         Vector3 position;
         position.x = -width + Random.Range(1f, (width * 2) - 2f);
         position.y = -height + Random.Range(1f, (height * 2) - 2f);
         position.z = -1f;
         Egg spike = null;
 
         spike = Instantiate(spikePrefab, position, Quaternion.identity).GetComponent<Egg>();
 
         spikes.Add(spike);
     }
 
     void KillOldEggs()
     {
         foreach (Egg egg in eggs)
         {
             Destroy(egg.gameObject);
         }
         eggs.Clear();
     }
 
     void KillOldSpikes()
     {
         foreach (Egg spike in spikes)
         {
             Destroy(spike.gameObject);
 
         }
         spikes.Clear();
     }
 
 }