Only error comes when try to build for android not for windows?

Code:

 var a:UI.Image;
 var b:Sprite;
 var c:Sprite;
 function Start () {
     a=GetComponent.<UI.Image>();
 }

 function Clicked() {
 
     a.sprite=b;
 }
     function Released(){
 
     a.sprite=c;
 }

error: {1}NullReferenceException: Object reference not set to an instance of an object

{2}Error building Player: NullReferenceException: Object reference not set to an instance of an object

i don't think its a problem in code.

I have added sdk tools and jdk in the external tools section.