Accesing textbox properties inside MoveClip

I get this problem very often it says, that i need to include the clip on on movieclip event, i don understand why.

i have a movieclip called question, and a textbox called boxText. i am assiging them the correct instance names.

the code is

this.question.boxText.text = "This is a sample text";

and it gives me a stupid error.

Could anyone help me here