Need help checking if an item slot is filled

Hello everybody, so I have an issue with some code my friend wrote. So my issue is figuring out how to activate a Power up once it has been stored into a slot, via a button press. I don't necessarily know where to look and have looked at this code over and over to try to figure it out but I may just not be focusing in the right area. there is a piece of commented out code that i wrote to try to piece it together but im not sure that it is the right code.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using Rewired;
 
 public class TestController : MonoBehaviour {
     private PowerUp[] StoredPowerUps = new PowerUp[4];
     private int NumberStoredPowerUps = 0;
     public PowerUp ActivePowerUp;
     public bool IsShuffling = false;
     private float ShuffleTimeOut = 5f;
     private float CurrentTimer = 0f;
     public PowerUp[] ShuffleList;
     public Image ShuffleIcon;
     public float ShuffleDelay = 0.05f;
     public float CurrentShuffleDelay = 0f;
     private int ShuffleIndex = -1;
     [SerializeField]
     private int PlayerId;
     private Player player;
     
     
     // Use this for initialization
     void Start () {
         player = ReInput.players.GetPlayer (PlayerId);
     }
     
     // Update is called once per frame
     void Update () {
         
         if (IsShuffling) {
             if(ShuffleIndex == -1){
                 ShuffleIndex = Random.Range(0,ShuffleList.Length);
             }
             CurrentShuffleDelay += Time.deltaTime;
             if(CurrentShuffleDelay >= ShuffleDelay){
                 CurrentShuffleDelay = 0f;
                 ShuffleIndex++;
                 if(ShuffleIndex >= ShuffleList.Length){
                     ShuffleIndex = 0;
                 }
                 ShuffleIcon.sprite = ShuffleList[ShuffleIndex].image;
             }
             CurrentTimer += Time.deltaTime;
             if(player.GetButton("StopPower") || CurrentTimer >= ShuffleTimeOut){
                 IsShuffling = false;
                 Debug.Log("S");
                 if(ShuffleList[ShuffleIndex].autofire){
                     ActivePowerUp = ShuffleList[ShuffleIndex];
                     Debug.Log("Activated "+ActivePowerUp.name);
                 }else{
                     int StoredIndex = NumberStoredPowerUps++;
                     if(StoredIndex < 4){
                         StoredPowerUps[StoredIndex] = ShuffleList[ShuffleIndex];
                         Debug.Log("Stored "+StoredPowerUps[StoredIndex].name+" in slot "+StoredIndex);
                     }
                 }
                 ShuffleList = null;
                 ShuffleIndex = -1;
                 CurrentTimer = 0f;
                 
             }
             /*
             if(player.GetButton("Power1") && StoredPowerUps[0]){
                 ActivePowerUp = ShuffleList[ShuffleIndex];
                 StoredPowerUps[StoredIndex] = ShuffleList[ShuffleIndex];
             }
             */
         }                
     }
 }