Loading external SWF in AS3

Hello

I am getting really annoyed with AS3, I know it is probably better for all you coders, but I cannot for the life of my find out how to load an external SWF through the clicking of a button in AS3.

import flash.net.URLRequest;
import flash.events.;
import flash.display.
;

var PhysTestRequest:URLRequest = new URLRequest(“PhysTest.swf”);
var PhysTestLoader:Loader = new Loader();

inv_btn.addEventListener(MouseEvent.MOUSE_DOWN, newvid);

function newvid(event:MouseEvent):void
{
trace(“you clicked me”);
PhysTestLoader.load(PhysTestRequest);
addChild(PhysTestLoader);

}
That is the code that I have cobbled together and I am just getting this error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at General::Input()
at Main()

Why is this?

If anyone could help me, I would be eternally grateful.

Thanks

Stefan