NullReferenceException?

Hi.
I want to get the Ball.cs component that is attached to the Ball object.
And I want to change ball rigidbody velocity.
When I wrote code.error is displayed.In C# script.

NullReferenceException
UnityEngine.GameObject.GetComponent[Ball] ()

Code (csharp):

1.  
2. ---GameControll.cs---------
3. void Start () {
4.     Instantiate(Ballprefab,ballpos,Quaternion.identity);
5. }
6. ---ball.cs---------
7. private Ball ball;
8.  
9. void Start () {
10.     ball = GameObject.Find("Ball(Clone)").GetComponent<Ball>();
11. }
12. void Example() {
13.     ball.rigidbody.velocity /= 1.2f;
14. }
15.  

Please help me.
Thank you.

 

Code (csharp):

1. ---GameControll.cs---------
2.  
3. void Start () {
4.  
5.     Instantiate(Ballprefab,ballpos,Quaternion.identity);
6.  
7. }
8.  
9. ---ball.cs---------
10.  
11. private Ball ball;
12.  
13.  
14.  
15. void Start () {
16.  
17.     ballObject = GameObject.Find("Ball(Clone)");
18.     ball = ballObject.GetComponent<Ball>();
19.  
20. }
21.  
22. void Example() {
23.  
24.     ball.rigidbody.velocity /= 1.2f;
25.  
26. }

 

Thank you! tatelax!