When ball hits wall bottom for 15th time , intstead of ad i get error : NullReferenceException: Object reference not set to an instance of an object BallScript.Start () (at Assets/BallScript.cs:44)

using UnityEngine; using UnityEngine.SceneManagement; using UnityEngine.UI; using UnityEngine.Advertisements;

public class BallScript : MonoBehaviour {

 private ManagerAds ads;

 public Vector2 startForce;

 static int times = 0;

 public Text goverText;

 public Vector3[] positions;

 public Rigidbody2D rb;

 void Start()
 {
     int randomNumber = Random.Range(0, positions.Length);
     transform.position = positions[randomNumber];
     times = PlayerPrefs.GetInt("Played:", times);
     rb.AddForce(startForce, ForceMode2D.Impulse);
     ads = GameObject.FindObjectOfType<ManagerAds>();


 }

 void Update()
 {

     PlayerPrefs.SetInt("Played:", times);
     goverText.text = "Played:" + times.ToString();


 }

 void OnCollisionEnter2D(Collision2D other)
 {
     if (other.gameObject.name == "wall_bottom")
     {

         times++;

         if (times == 15 )
         {
             Debug.Log("ShowAd");
             ads.PlayRewardedAd();

         }
        

         SceneManager.LoadScene("wfvv");


     }

 }

}