TypeError: #1009 Issues

Question

Okay. I'm trying to make a popup alert modal window thing with an external class. I have a simple swf set up with just some random sample text, a rectangle, and a button (just so I can tell if it's working or not). What I want to happen is for the "alert" class to initialize upon startup of the swf and then when I click the button (instance: btn), the alert will show up. If I get rid of all AS in the swf and set alert as the document class, it shows up just fine, so I think it should be working, but for some reason I get this instead when I try the button function:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at alert$cinit()
at global$init()
at alertTester_fla::MainTimeline/frame1()
Here's the code in my alertTester swf:
import alert;
alert.msgAlert = "This is me testing stuff out.";
btn.addEventListener(MouseEvent.CLICK, displayAlert);
function displayAlert(e:MouseEvent):void
     alert.showAlert();
And here's the code in alert.as:
package  {
     import flash.display.MovieClip;
     import flash.display.*;
     import flash.events.*;
     import flashx.textLayout.formats.Float;
     import flash.text.TextField;
     import flash.geom.Matrix;
     import flash.text.*;
     public class alert extends MovieClip {
          private static var stage:Stage = null;
          private static var bkgd:Sprite;
          private static var msgBox:Sprite;
          private static var msg:TextField;
          public static var msgAlert:String = "Test.";
          //for alert box itself
          public static var rectW:int = 200;
          public static var rectH:int = 150;
          private static var rectX:Number = (stage.stageWidth/2) - (rectW/2);
          private static var rectY:Number = (stage.stageHeight/2) - (rectH/2);
          public function alert() {
            addEventListener(Event.ADDED_TO_STAGE, added);
        private function added(event:Event):void {
            init(stage);
          private function init(stageRef:Stage):void
               stage = stageRef;
               trace("Initialized!");
          public static function showAlert():void {
               if (stage == null) {
                    trace("Alert class has not been initialized!");
                    return;
               //initialize
               bkgd = new Sprite();
               msgBox = new MovieClip();
               msg = new TextField();
               //assign content
               bkgd = createBkgd();
               msgBox = createBox();
               msgText(msgAlert);
               //add children
               msgBox.addChild(msg);
               bkgd.addChild(msgBox);
               stage.addChild(bkgd);
          private static function createBkgd() : Sprite
               //setup/initialize
               var overlay:Sprite = new Sprite();
               //make rectangle/cover stage
               overlay.graphics.beginFill(0x929292);
               overlay.graphics.drawRect(0,0,stage.stageWidth,stage.stageHeight);
               overlay.graphics.endFill();
               overlay.alpha = .85;
               //return
               return overlay;
          private static function createBox() : Sprite
               //setup/initialize necessary variables
               var box:Sprite = new Sprite();
               var colors:Array = new Array(0xFFFFFF, 0xE1E1E1);
               var alphas:Array = new Array(1, 1);
               var ratios:Array = new Array(0,125);
               var mat:Matrix = new Matrix();
               //make rectangle/alert box
               mat.createGradientBox(rectW,rectH,(Math.PI/2),rectX,rectY);
               box.graphics.beginGradientFill(GradientType.LINEAR, colors, alphas, ratios, mat);
               box.graphics.drawRoundRect(rectX,rectY,rectW,rectH,25,25);
               box.graphics.endFill();
               //return
               return box;
          private static function msgText(alertMsg:String):void
               //create formatter and font
               var myFormat:TextFormat = new TextFormat();
               var myFont = new Font();
               //format formatter
               myFormat.size = 14;
               myFormat.align = TextFormatAlign.CENTER;
               myFormat.font = "Arial";
               //declare and assign message variable
               var msgWords:String = alertMsg;
               //set text field properties
               msg.autoSize = TextFieldAutoSize.CENTER;
               msg.defaultTextFormat = myFormat;
               msg.background = false;
               msg.border = false;
               msg.selectable = false;
               msg.type = TextFieldType.DYNAMIC;
               msg.textColor = 0x000000;
               msg.antiAliasType = AntiAliasType.ADVANCED;
               msg.embedFonts = true;
               msg.text = msgWords;
               //set position
               msg.x = rectX + rectW/2 - msg.width/2;
               msg.y = rectY + rectH/5;
I originally had the "init" function as a public static function so I could do "alert.init(stage);" from the swf, but it gave me this same error.
Thanks in advance for any help you can offer. =\

Answer 1

First, you cannot do that in the class header. stage is available ONLY when object is added to display list. This, I believe, is the cause for the error.
private static var rectX:Number = (stage.stageWidth/2) - (rectW/2);
private static var rectY:Number = (stage.stageHeight/2) - (rectH/2);
Second, stage is a property of MovieClip, so the following line at least unnecessary:
private static var stage:Stage = null;