prefab shooting error...help

Hi all, I followed a lot of tutorial but none resolved my problem. I want to shoot an arrow. I made a spawnPoint empty gameObject, I made my arrow prefab with a cube on wich I added a rigidbody and I linked this script to my spawnPoint...

 using UnityEngine;
 using System.Collections;
 
 public class prefabShooting : MonoBehaviour {
     public Rigidbody projectilePrefab;
     public int projectileSpeed;
  
     public void Update(){
         if(Input.GetKey("f")){
             Rigidbody instantiatedProjectile;
 
             projectileSpeed=10;
             instantiatedProjectile=Instantiate(projectilePrefab, transform.position, transform.rotation) as Rigidbody;
             instantiatedProjectile.velocity=transform.TransformDirection (new Vector3(0,0,projectileSpeed));
             //Destroy(clone.gameObject, 5);
         }
     }
 } 

I press play and the game start but when I press F the game pause and send me this error...

UnassignedReferenceException: The variable projectilePrefab of prefabShooting has not been assigned. You probably need to assign the projectilePrefab variable of the prefabShooting script in the inspector.

but I affected it. When I unpause the game, an arrow appear and is shot like supposed too but when I press F again the game pause again and the process start over again and again indefinitely until I stop it... Can somebody help me here before I turn crazy.