Adding instance to stage from a movieclip linked class

programming flash
1

Hi,
I was trying to add an instance I created in a class to the main
stage, like this:

var blackD:MovieClip = new blackboardinfo();
Stage.addChild(blackD);

it was working fine, however, after I added preload page, which will now
load everything else as external swf, i began getting this error
TypeError: Error #1009: Cannot access a property or method of a null object reference.

I tried to modify the code to this based on the similar questions i found online

var mc:MovieClip = new MovieClip();
addChild(mc);
var _stage:Stage = mc.stage;
_stage.addChild(blackD);

I still get the same error, I know it is because “stage”, but if I dont use stage, how
can I add an instance on top of everything? thanks.