Object reference not set to an instance of an object

I'm new in unity and my code showing me this error

NullReferenceException: Object reference not set to an instance of an object ShootScript.Update () (at Assets/Scripts/ShootScript.cs:30)

please tell me how to fix this.

 using UnityEngine;
 using System.Collections;
 
 public class ShootScript : MonoBehaviour {
 
     Rigidbody arrowRigidBody;
 
     public int shootSpeed;
     public bool shootStart;
     public float xForce;
     public float yForce;
     public float movementSpeed;
     public float distance;
 
 
 
     void Start()
     {
         arrowRigidBody = GetComponent<Rigidbody> ();
     }
 
     void Update()
     {
         
         if (shootStart)
         {
             //float xtemp = Input.acceleration.x*xForce;
             //float ytemp = Input.acceleration.y*yForce;
             // ArrowRigidBody.velocity = new Vector3 (-xtemp,-ytemp,shootSpeed);
             arrowRigidBody.            transform.position = transform.position + Camera .main.transform.forward * distance * Time.deltaTime;
 
 
 
         }
     }
 
     public void stop()
     {
 
         arrowRigidBody.velocity=new Vector3 (0,0,0);
         shootStart = false;
 
     }
 
 
 
     public void setBoolea()
     {
 
         shootStart = true;
     }
       
 
 }