AS3 loading

Greeting to all!

I learning OOP in AS3 and came across something that has me bound!

I am trying to load a external movieclip on to the stage from a movieclip. I am not getting any error but the file is not loading on to the stage. I have been getting the following message:
TypeError: Error #1009: Cannot access a property or method of a null object reference.

The following is the script that I have inside the movieclip:

test1_btn.addEventListener(MouseEvent.CLICK, myButtonFunction);
function myButtonFunction(event: MouseEvent) {
var request:URLRequest = new URLRequest(“happy.swf”);
var loader:Loader = new Loader();
addChild(loader);
}

Am I missing something?