Accessing a named movieclip in a movieclip using as3

Hi,

I have created a new instance of a movieclip and named it in as3.

I want to put this in another movieclip. How does this effect the reference because everything seems fine while the dynamic movieclip is just dealing with the stage. As soon as I put it in a movieclip, I get:

Error #1009: Cannot access a property or method of a null object reference.
    at com::Gallery/instancesOut()

Heres part of my code:

for(var i:int = 0; i < 20; i++)
{
    var transition:mcTransition = new mcTransition();

    addChild(transition);
    transition.name = "transition_mc" + i;
    transition.x = i * 100;

I can reference using the string name but if I put the addchild in a movieclip it errors

Transition_mc.addChild(varTransition);

Please help

Thanks