Checking if first item in array has been destroyed

 if (updateEnemyList.Count > 0 && updateEnemyList [0] == null)

The above check will always return false if it's done in the same update loop cycle (Not necessarily the same script) as the element is destroyed because when an object is destroyed, its reference is not to null immediately. Rather it is done just before rendering the corresponding frame.

Here is a snippet to test out the behaviour:

 void Start () {    
         StartCoroutine(CreateAndDestroy());
     }
     
     IEnumerator CreateAndDestroy(){
         //create
         print("Creating Clone");
         clone = Instantiate<Transform>(obj);
         yield return new WaitForSeconds(2);
         //destroy
         print("Destroying Clone");
         Destroy(clone.gameObject);
         //Cheking immediately for null (should return false)
         print("Checking null");
         print(clone==null);
         
         yield return null;
         //wait till next frame and check again
         print("Checking null on next frame (should return true)");
         print(clone==null);
     }

One possible solution will be to delay your null check till the next frame.

Another solution (which I personally think could be simpler if I think through a bit more) is detailed below:

• Create a custom behaviour which extends the MonoBehavior class and add a custom Destroy method and an isDestroyed field which is set immediately after Destroy executes.

• Let all your subsequent behaviors extend that class and call the custom destroy method to destroy other Behaviors that extend custom Bevahior.

• check IsDestroyed property (instead of a null check) to query if an object has been destroyed.

Code snippets for classes are below:

Custom base behavior classs:

 public class MyBehavior : MonoBehaviour
 {
     // Property to check after calling Destroy
     private bool _isDestroyed = false;
     public bool IsDestroyed{ get {
             return _isDestroyed;
         } 
     }
     
     //custom Destroy
     public static void Destroy(MyBehavior obj, bool destoryGameObject = false){
     
         if(destoryGameObject){
             Destroy(obj.gameObject);
         }
         else{
             Destroy(obj);
         }
         obj._isDestroyed = true;
     }
 }

Object that extends custom Behaviour:

 public class DestroyableObject : MyBehavior {
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }

Instantiating and Destroying our Object (Similar to previous example but replaces null check with isDestroyed() ):

 IEnumerator CreateAndDestroy(){
         //create
         print("Creating Clone");
         clone = Instantiate<DestroyableObject>(obj);
         yield return new WaitForSeconds(2);
         //destroy
         print("Destroying Clone");
         Destroy(clone,true);
         //Cheking immediately for IsDestroyed (should return true)
         print("Checking IsDestroyed");
         print(clone.IsDestroyed);
         
         yield return null;
         //wait till next frame and check again
         print("Checking null on next frame");
         print(clone==null);
     }
     

Hope this helps