MissingReferenceException: The object of type 'GameObject' has been destroyed..(ERROR)

I am making a game which is really similar to flappy bird for my school project. I successfully made the pipes regenerate but when I trying to destroy pipes that error appears and my pipes just suddenly stop generate.
This is my script for generating pipes

using UnityEngine;
using System.Collections;

public class Pipes : MonoBehaviour {
    public GameObject thepipes;

    public float maxHeight;
    public float minHeight;

    public float DistanceBetween;
   
    public Transform generatorThreshold;

    // Use this for initialization
    void Start () {
        transform.position = new Vector3 (transform.position.x + DistanceBetween, transform.position.y, transform.position.z);
    }
  
    // Update is called once per frame
    void Update () {
        if (transform.position.x < generatorThreshold.position.x)
           {
            Instantiate (thepipes, new Vector3 (transform.position.x, Random.Range (minHeight, maxHeight), transform.position.z), transform.rotation);
            transform.position = new Vector3 (transform.position.x + DistanceBetween, transform.position.y, transform.position.z);
        }    
    }
}

And this is my script for destroying pipes

using UnityEngine;
using System.Collections;

public class Destroy : MonoBehaviour {
    public Transform destructionPoint;

    // Use this for initialization
    void Start () {
        destructionPoint = GameObject.Find ("Destruction Point").transform;
    }

    // Update is called once per frame
    void Update () {
        if (transform.position.x < destructionPoint .position.x)
        {
            Destroy (gameObject);
        }
    }
}

And this error keeps occur when I press play.
I really need help for my school project immediately…!! Thank you!!
If I made any mistake, please point it out so I can fix it…:wink:
Oh, I am refering a tutorial by gameplusjames in Youtube. This is the URL

check to see if its null or not null before instantiating ?