Very Strange Issue with Error #1009

programming flash
1

I’m trying to load an external .swf using UILoader. If the .swf doesn’t have any code, it loads fine. But if I add any code, even only comments. It gives me this error.

 
TypeError: Error #1009: Cannot access a property or method of a null object reference.
 at main_fla::MainTimeline/__setProp_BackgroundLoader_Scene1_BG_0()
 at main_fla::MainTimeline()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
 at main_fla::MainTimeline/LoadComplete()
 at flash.events::EventDispatcher/dispatchEventFunction()
 at flash.events::EventDispatcher/dispatchEvent()
 at flash.net::URLLoader/onComplete()

Here is my project.
http://kritphong.co.cc/downloads/project.zip