MissingReferenceException?

"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. CustomControlScript.Update () (at Assets/Scripts/CustomControlScript.cs:51)."

This is the error I'm getting when this block of code:

     if(Input.GetKeyDown(KeyCode.S))
     {
         List<GameObject> newList = new List<GameObject>();
         foreach(GameObject b in MySingleton.Instance.MyList)
         {
             AnotherScript script = (AnotherScript)b.GetComponent(typeof(AnotherScript));//Error Right Here    
             GameObject newB = script.DoSomething();
             newList.Add(newB);
         }
         foreach(GameObject b in newList)
         {
             MySingleton.Instance.MyList.Add(b);
             newList = null;
         }
     }
 

Is executed ONLY AFTER this block of code in MySingleton is executed (If "S" is pressed before this function is executed, all is well):

 public void EmptyList()
 {
         foreach(GameObject obj in MyList)
         {
             Destroy(obj);
         }
         for (int i = 0; i < MyList.Count; i++)
         {
     
             MyList.RemoveAt(i);
         }
 }

How might I fix this issue? What I want to do is be able to clear my list whenever I want to and add new items to it afterward.