NullReferenceException: Object reference not set to an instance of an object ?

 #pragma strict
 
 import UnityEngine.UI;
 import System;
 import System.IO;
 
 public class PlayerBorder extends MonoBehaviour {
 
     public var player: ePlayer;
     public var score : ScoreUI;
  
 
     function OnCollisionEnter(col : Collision) {
         var ball : Ball = col.gameObject.GetComponent.<Ball>();
         if (ball != null){
             ball.transform.position = new Vector3(0.0, 1.0, 0.0);
 
             if (player == ePlayer.Right){
                 score.scorePlayerLeft = score.scorePlayerLeft+1;
             }
             else if (player == ePlayer.Left){
                 score.scorePlayerRight = score.scorePlayerRight+ 1;
             }
         }
     }
   }

i have this problem can anyone explain how to fix it? it always happen when somebody score....

NullReferenceException: Object reference not set to an instance of an object PlayerBorder.OnCollisionEnter (UnityEngine.Collision col) (at Assets/PlayerBorder.js:19)