Missing Reference exception when accessing a GameObject in Unity

I have been making a C# script that attacks the player in Unity. However, with the following code I got the following error message:

"The object of type 'GameObject' has been destroyed but you are still trying to access it.

Here is my code:

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

public class attackPlayer : MonoBehaviour {

    GameObject Enemy;
    GameObject Player;

    void Start ()
    {
        Enemy =  GameObject.FindWithTag ("Enemy");
        Player = GameObject.FindWithTag ("Player");
    }

    void Update ()
    {
        var heading = Enemy.transform.position.x - Player.transform.position.x;
        if (heading > 0.0f && heading < 5.0f) 
        {
            Explode ();
        }
    }

    void Explode() 
    {
        var exp = GetComponent <ParticleSystem>();
        exp.Play ();

        Destroy (gameObject,exp.main.duration);
        Destroy(Player);
    }

    void OnCollisionEnter2D (Collision2D col)
    {
        if (col.gameObject.tag == "Player") 
        {
            Explode ();
        }
    }
}