ActionScript 3.0 :: Keep Getting Error 1009 Until SWF Movie Window Closed?

I have created a simple slideshow of thirteen frames. The slideshow script starts at Frame 2. This is because I have inserted a preloader in Frame 1. When I output my SWF and upload it to the web, it pulls down without incident. However, when I "Simulate Download" locally, Flash says this in the Output window:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at AS3GallerywPreloader_fla::MainTimeline/updatePreloader()
[AS3GallerywPreloader_fla.MainTimeline::frame1:9]
And it keeps writing this error message, ad infinitum, until I close the SWF movie window!

Here is the preloader script:
stop();
loaderInfo.addEventListener (ProgressEvent.PROGRESS, updatePreloader);
function updatePreloader (evtObj: ProgressEvent): void{
var percent:Number = Math.floor((evtObj.bytesLoaded*100) /evtObj.bytesTotal);
preloader_txt.text = percent+"%";
if (percent==100) {
nextFrame ();
}}

I should note that the slideshow portion functions as it should (the Back and Forward buttons advance frame by frame). In essence, the whole movie works. But I suspect I that error code is a bad thing to have. I ran the "Debug" feature (Debug-->Debug Movie), but got no apparent indication of a bad line of code.