问为什么我在一次又一次启动协程时得到MissingReferenceException异常？

EN

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

public class GenerateWalls : MonoBehaviour
{
    public GameObject gameObjectToRaise;
    public float duration;
    public Vector3 raiseAmount;
    public bool go = false;
    public Color[] colors = new Color[4];
    public bool randomColors = false;

    private GameObject objtoraise;
    private GameObject[] walls;
    private bool scaleOver = false;

    private void Start()
    {
        Init();

        ColorWalls();

        // The z Axis must be minimum 1 or any value above 0 could be also 0.1f
        // but it's better to keep it minimum as 1 by default.
        if (raiseAmount.z < 1)
        {
            raiseAmount.z = 1f;
        }

        if (go)
        {
            StartCoroutine(ScaleOverSeconds(objtoraise, new Vector3(raiseAmount.x, raiseAmount.y,
                raiseAmount.z), duration));
        }
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.R))
        {
            //if (scaleOver)
            //{
            if (objtoraise != null)
            {
                if (raiseAmount.z < 1)
                {
                    raiseAmount.z = 1f;
                }


                Destroy(objtoraise);

                Init();

                ColorWalls();

                StartCoroutine(ScaleOverSeconds(objtoraise, new Vector3(raiseAmount.x, raiseAmount.y,
                    raiseAmount.z), duration));

                scaleOver = false;
                //}
            }
        }
    }

    private void Init()
    {
        objtoraise = Instantiate(gameObjectToRaise);
        objtoraise.name = "Walls";

        walls = GameObject.FindGameObjectsWithTag("Wall");
    }

    public IEnumerator ScaleOverSeconds(GameObject objectToScale, Vector3 scaleTo, float seconds)
    {
        float elapsedTime = 0;
        Vector3 startingScale = objectToScale.transform.localScale;
        while (elapsedTime < seconds)
        {
            objectToScale.transform.localScale = Vector3.Lerp(startingScale, scaleTo, (elapsedTime / seconds));
            elapsedTime += Time.deltaTime;
            yield return new WaitForEndOfFrame();
        }

        objectToScale.transform.localScale = scaleTo;

        scaleOver = true;
    }

    private void ColorWalls()
    {
        for (int i = 0; i < walls.Length; i++)
        {
            if (randomColors)
            {
                walls[i].transform.GetChild(0).GetComponent<Renderer>().material.color
                    = GetRandomColour32();
            }
            else
            {
                walls[i].transform.GetChild(0).GetComponent<Renderer>().material.color = colors[i];
            }
        }
    }

    private Color32 GetRandomColour32()
    {
        //using Color32
        return new Color32(
          (byte)UnityEngine.Random.Range(0, 255), //Red
          (byte)UnityEngine.Random.Range(0, 255), //Green
          (byte)UnityEngine.Random.Range(0, 255), //Blue
          255 //Alpha (transparency)
        );
    }
}

objectToScale.transform.localScale = Vector3.Lerp(startingScale, scaleTo, (elapsedTime / seconds));