NullReferenceException!! Please Give a Hand

Hi, all Unity Hero, I have a question to execute the following script which is the unlock system by PlayerPrefs, NGUI is used as plugins, The Unity shows me the following

,"NullReferenceException unlock system1.Update () (at Assets/Scripts/unlock system1.js:19)"

and I have no any clue to solve it, please kindly give me a hand.

 #pragma strict
 
 var levelReached1 : int =0;
 var levelReached2 : int =0;
 var levelReached3 : int =0;
 
 
 levelReached1 = PlayerPrefs.GetInt("1");
 levelReached2 = PlayerPrefs.GetInt("2");
 levelReached3 = PlayerPrefs.GetInt("3");
 
 
 
 
 function Update(){
 
 var S1B : UIButton = GameObject.Find("S1 Button").GetComponent(UIButton);
 var S2B : UIButton = GameObject.Find("S2 Button").GetComponent(UIButton);
 var S3B : UIButton = GameObject.Find("S3 Button").GetComponent(UIButton);     <-- Here
 
 
 
 if(levelReached1 == 1)
 {S1B.active = true;
  S2B.active = true;}
 else if (levelReached1 ==0)
 {S1B.active = true;
  S2B.active = false;}
  
  if(levelReached2 == 2)
  {S2B.active = true;
  S3B.active = true;}
 else if (levelReached2 ==0)
 {S2B.active = true;
  S3B.active = false;}
  
  if(levelReached3 == 3)
  {}
 else if (levelReached3 ==0)
 {}
  
  
  }