NullReferenceException

I instantiate a prefab into a variable, but on line 37 I get a nullreferenceexception. Sorry for the post being so rushed, but I’m short on time. Here is my code:
#pragma strict
public var script : UnityStandardAssets.Characters.FirstPerson.FirstPersonController;
var Money = 100;
public var GlassPrefab : Transform;
private var Rotation;
private var ShowBuildMenu = false;
private var notBuilt = false;
//private var Preview : GameObject;
var CanvasObject : GameObject;
private var BuildSelection = “Glass 1x1”;
//var script = gameObject.GetComponent(“MouseLook”);
function Start () {
Screen.lockCursor = true;
}

function FixedUpdate () {
if(Input.GetKeyDown(KeyCode.E)){
ShowBuildMenu = !ShowBuildMenu;
script.enabled = !ShowBuildMenu;
CanvasObject.active = ShowBuildMenu;
Screen.lockCursor = !ShowBuildMenu;
}
if(ShowBuildMenu == true){
Time.timeScale = 0;
}
else{
Time.timeScale = 1;
}
if(Input.GetKeyDown(KeyCode.Q) && notBuilt == false){
if(BuildSelection == "Glass 1x1"){
Rotation = Quaternion.Euler(90, transform.rotation.y, transform.rotation.z);
var Preview = Instantiate(GlassPrefab, transform.position + transform.forward * 4, Rotation) : GameObject;
notBuilt = true;
}
}
if(notBuilt){
Preview.transform.position = transform.position + (transform.forward * 4);
if(Input.GetKeyDown(KeyCode.Q)){
notBuilt = false;
Money = Money - 25;
}
}
}
function OnGUI(){
GUI.Label(new Rect(10, 10, 200, 50), "Money: "+Money);
}

function BuildGlassPane(){
BuildSelection = "Glass 1x1";
}

The error I get is this:

NullReferenceException: Object reference not set to an instance of an object
BuilderScript.Update () (at Assets/BuilderScript.js:37)

The instantiation happens on line 32.

You are ignoring the global Preview variable, because you are declaring a new local variable Preview when you instantiate the Object. I am not sure if this is your issue or not, but you should double check this issue.

You never instantiated Preview.