Object reference not set to an instance of an object C#

Hi guys,

I am currently trying to code a script that will allow me to spawn a prefab and add force, I have got half way, allowing it to spawn in. my issue arises when I try add force to the spawned prefab. Have had an issue complying my prototype code, but when I run it I have an error appear :

Object reference not set to an instance of an object Gun1ShootScript.Update () (at Assets/Scripts/Gun1ShootScript.cs:20)

From what I have looked into It looks like it is struggling to find the prefab, My question is how should I apply force to this?

 using UnityEngine;
     using System.Collections;
     
     public class Gun1ShootScript : MonoBehaviour {
     
         public GameObject Bullet1;
         public float speed = 20;
         Rigidbody RB;
         // Use this for initialization
         void Start () {
         
         }
     
         // Update is called once per frame
         void Update() {
     
             if (Input.GetButtonDown("Fire1"))
             {
                 Rigidbody instantiatedProjectile = Instantiate(Bullet1, transform.position, transform.rotation) as Rigidbody;
                 instantiatedProjectile.velocity = transform.TransformDirection(new Vector3(speed, 0f, 0f));
     
          
             }
         }
     }