Object Pool object reference is null?

Hey. So I'm new to the whole Unity thing and basically for a college assignment I have to assign a design pattern to my final year project. Since I'm doing a game in Unity I need to apply it to this. I was going to apply it to different spells that you can select to shoot different objects.

So I followed a YouTube tutorial on how to create a ObjectPool and I thought everything was going okay, however when I start the game, upon clicking, nothing emits, and I also get an error at the bottom saying "Object reference was not set to an instance of an object".

I created a prefab with a sphere in it. I created 2 scripts, 1 called Object Pool, the other called Projectile. My Camera has its own script called FirstPersonController that is used for movement. In the ObjectPool script I detect if the user has pushed down the mouse button, then I call the ActivatedProjectile method.

EDIT

I get 2 types of error. The first appears only once at the start of the code:

ArgumentException: The thing you want to instantiate is null. UnityEngine.Object.CheckNullArgument (System.Object arg, System.String message) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/UnityEngineObject.cs:104) UnityEngine.Object.Instantiate (UnityEngine.Object original) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/UnityEngineObject.cs:90) ObjectPool.InstantiateProjectiles () (at Assets/Scripts/ObjectPool.cs:28) ObjectPool.Start () (at Assets/Scripts/ObjectPool.cs:12)

The other appears multiple times as follows:

NullReferenceException: Object reference not set to an instance of an object ObjectPool.ActivateProjectile () (at Assets/Scripts/ObjectPool.cs:37) ObjectPool.Update () (at Assets/Scripts/ObjectPool.cs:20)

Here is the code I have for ObjectPool class.

 using UnityEngine;
 using System.Collections;
 
 public class ObjectPool : MonoBehaviour {
 
     GameObject[] projectiles = null;
     public int numberOfProjectilesToCreate = 0;
 
     // Use this for initialization
     void Start () {
         projectiles = new GameObject[numberOfProjectilesToCreate];
         InstantiateProjectiles ();
     }
     
     // Update is called once per frame
     void Update () 
     {
         if(Input.GetMouseButton(0))
         {
             ActivateProjectile();
         }
     }
 
     private void InstantiateProjectiles()
     {
         for (int i = 0; i < numberOfProjectilesToCreate; i++) 
         {
             projectiles[i] = Instantiate(Resources.Load("Prefabs/prefab_projectile")) as GameObject;
             projectiles[i].SetActive(false);
         }
     }
 
     private void ActivateProjectile()
     {
         for (int i = 0; i < numberOfProjectilesToCreate; i++) 
         {
             if(projectiles[i].activeSelf == false)
             {
                 projectiles[i].SetActive(true);
                 projectiles[i].GetComponent<Projectile>().Activated();
             }
         }
     }
 }
 

Anybody have any ideas on what I'm doing wrong?