Access to a textfield in a movieclip on different frames

1, Seems like a standard job, but somehow this doesnt work:
Movieclip called “Button” lies in the library.
I instantiate it in code:

var button:MovieClip = new Button();

I have a dynamic textfield with an instance name of “description_text” in it, but if I try to access it (e.g: change the text), I get an error. This is how I do it:

(button.getChildByName("description_text")as TextField).text = "Success";

Cannot access a property or method of a null object reference.
Why is this not working?
2, I know it’s easier to just have the movieclip invisible on all frames except the ones that I need it. I’m just curious if its possible at all to selectively choose single frames and access their content.