Problem with addEventListener

Question

Created preloader, and after flash loaded want stop on label 'welcomeSite'. And on that label will have button, after click this button what play code from function 'irisDescription'.
But see error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at FlashSite/flashSiteLoading()[FlashSite.as:28]
code:
package
     import flash.display.MovieClip;
     import flash.events.*;
     import flash.text.*;
     public class FlashSite extends MovieClip 
          //constructor
          public function FlashSite()
               stop();
               addEventListener(Event.ENTER_FRAME, flashSiteLoading);
          public function flashSiteLoading(event:Event)
               var mcBytesLoaded:int=this.root.loaderInfo.bytesLoaded;
               var mcBytesTotal:int=this.root.loaderInfo.bytesTotal;
               var mcKLoaded:int=mcBytesLoaded/1024;
               var mcKTotal:int=mcBytesTotal/1024;
               flashSiteLoading_txt.text="Loading: "+mcKLoaded+"Kb of "+mcKTotal+"Kb";
               if (mcBytesLoaded>=mcBytesTotal)
                    removeEventListener(Event.ENTER_FRAME, flashSiteLoading);
                    gotoAndPlay('welcomeSite');
                    irisBtn.addEventListener(MouseEvent.CLICK, irisDescription);
          public function irisDescription(event:MouseEvent):void
               trace("Iris description goes here.");
27 line is: irisBtn.addEventListener(MouseEvent.CLICK, irisDescription);
thanks for help

Answer 1

irisBtn isn't defined (in the scope of that class).
if that's your document class, irisBtn isn't in frame 1 of your main timeline.