destroying a unit and removing it from a list

Hey guys, Im making a simple rts game and when I have my player unit destroy an enemy unit is keeps returning erros and breaks the game. I do not know the right way to go about this. I have lists detect the enemies on the field. The player unit finds the closest one and attacks it. WHen its dead i use destroy object and some null exceptions but im still getting errors. Could you help? Thanks,

 public void Start () {

     StartCoroutine (CountEnemies()); //counts the amount of enemies that have been spawned

     StartCoroutine (DetectEnemies()); //this detects if the enemies are in range, and then moves to them

     
 }

 public void FixedUpdate () { //runs update at a fixed fps

     //attacking the enemy

     if (playerAttacking == true) {
         
         GameObject EnemyObject = EnemyList[minDistanceIndex]; //creates an object for that specific player unit                            

         if(EnemyObject != null){

         Enemy EnemyHealthDecrease = EnemyObject.GetComponent<Enemy> ();
         
         //    print (Time.time);
         
         if(Time.time>=coolDown){
             Debug.Log ("-----------------------the player hit an enemy unit!");//simple test for enemy attack



             if(EnemyHealthDecrease.health>0){
             EnemyHealthDecrease.health-=30;
             print("the units health is at" + EnemyHealthDecrease.health);
             coolDown = Time.time + attackRate;
             }
             if(EnemyHealthDecrease.health<=0){

                 Destroy(EnemyObject);
                 EnemyList.RemoveAt(minDistanceIndex);
                 
                     playerAttacking=false;
                 DistanceToEnemyList.Clear();
                 SortedDistanceToEnemyList.Clear();
                 
                 }
             
             }
         }
         
     }
 IEnumerator CountEnemies(){ //this is where the game detects the # enemies that have been spawned
     
         while (true) {

         GameObject[] EnemyArray = GameObject.FindGameObjectsWithTag("Enemy"); //creates an array populated with the enemies
         
         for(int i=0 ; i<EnemyArray.Length; i++){ //iterates through the array and adds all objects to the list
             if(EnemyArray[i]!=null){
             EnemyList.Insert(i, EnemyArray[i]); //adding to the list
             }
         }
         yield return new WaitForSeconds(1f);
     }
 }

 IEnumerator DetectEnemies(){

 while (true) {
 
         if(playerAttacking==false){

         for(int i=0; i< EnemyList.Count; i++){

             if(EnemyList[i] !=null){

             float distance = Vector3.Distance (transform.position, EnemyList[i].transform.position); //finds distance between enemy and player

             DistanceToEnemyList.Insert(i,distance); //inserts the distance values at the indicies

             SortedDistanceToEnemyList.Insert(i,distance); //this is the same as distancelist but will be sorted
             
             SortedDistanceToEnemyList.Sort();//sorts the list in ascending order

             //min distance is the first index of sorted list...
             }
         }
             if(SortedDistanceToEnemyList[0]!=null){
             minDistanceIndex = DistanceToEnemyList.IndexOf(SortedDistanceToEnemyList[0]);
             }
         }

          //this should give the index of the smallest distance

         if (DistanceToEnemyList[minDistanceIndex] <= 10 && minDistanceIndex!=null) { //if the distance is less than 10 between player unit and enemy unit                
             //index out of range?

             GameObject enemyObject = EnemyList[minDistanceIndex]; //creates an object for that specific player unit                            
             if(enemyObject!=null){
             AstarAIEnemy enemyObjectDetection = enemyObject.GetComponent<AstarAIEnemy> (); //gets astarai code for that unit

                     targetPosition = enemyObjectDetection.transform.position; //sets the enemy position to the pos of the player unit
                     
                     float distanceToTargetPos = Vector3.Distance (targetPosition, transform.position); //finds distance between the two units

                         if (distanceToTargetPos > 2 && distanceToTargetPos<=10) {
                             playerAttacking=false;//this makes it so the enemy stops attacking when player is far away
                             seeker.StartPath (transform.position, targetPosition, OnPathComplete); //if distance is more than 3 than travel towards its destination
                         }
                         if (distanceToTargetPos<2){
                             playerAttacking=true;
                         }
             }
         }//end of if distance < X

 //        Debug.Log("Check Enemy detection"); 
         //DistanceToEnemyList.Clear();
         //SortedDistanceToEnemyList.Clear();
         yield return new WaitForSeconds(1f);

     
     }

 }