Error: Object has been destroyed

When I try to run my game I get the following error:

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

Here is my code but to tell the truth I don't know why the problem occurs or what exactly causes the problem.

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

public class DisparoMovimiento : MonoBehaviour {

    public GameObject player;
    private Transform playerTrans;

    private Rigidbody2D DisparoRB;

    public float DisparoSpeed;

    public float bolitalife;


    void Awake(){
        DisparoRB = GetComponent<Rigidbody2D>();
        player = GameObject.FindGameObjectWithTag("Player"); 
        playerTrans = player.transform;
    }
    // Use this for initialization
    void Start () {
        if (playerTrans.localScale.x > 0){
            DisparoRB.velocity = new Vector2(DisparoSpeed, DisparoRB.velocity.y);
            transform.localScale = new Vector3(1, 1, 1);
        }
        else{
            DisparoRB.velocity = new Vector2(-DisparoSpeed, DisparoRB.velocity.y);
            transform.localScale = new Vector3(-1, 1, 1);
        }
    }

    // Update is called once per frame
    void Update(){
    }
}