MissingReferenceException: The object of type 'GameObject' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object.

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

public class enemyHealth : MonoBehaviour {

 public float enemyMaxHealth;
 public GameObject enemyDeathFX;
 public Slider enemySlider;

 public bool drops;
 public GameObject theDrops;

 float currentHealth;

 // Use this for initialization
 void Start () {
     currentHealth = enemyMaxHealth;
     enemySlider.maxValue = currentHealth;
     enemySlider.value = currentHealth;

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

 public void addDamage(float damage){
     enemySlider.gameObject.SetActive (true);
     currentHealth -= damage;
     enemySlider.value = currentHealth;

     if (currentHealth <= 0) makeDead ();
 }

 void makeDead(){
     Destroy (gameObject);
     Instantiate (enemyDeathFX, transform.position, transform.rotation);
     if (drops)
         Instantiate (theDrops, transform.position, transform.rotation);
 }

}