NullReferenceException Problem

I am getting an object reference not set issue, and I am a newbie coder. Joystick refers to a script named joystick.js. Any help would be great.
Here is the code:
#pragma strict

var Stick: Joystick; 
function Update ()
{

   transform.Translate(Vector2.up * Time.deltaTime * Stick.position.x);
    }

You’ve declared a variable name Stick, but have you assigned anything to it? You simplest way to do that is by dragging the joystick.js script to the Stick property in the editor.