Stage text field access from class

1

Hey,
A simple access from class question:

I have a dynamic text field on my stage, instance name: “lives_txt”
I want to populate this field from my Document class “Ball.as”

So in my Ball.as class I have tried: parent.lives_txt.text = String (lives);
where: public var lives:int = 3;

I get an error: access of possibily undefined property lives_mc through a reference with static type flash.display:DisplayObjectContainer.

What am I doing wrong?

Thanks