ActionScript 3.0 :: MyButton_btn Does Not React To Click Events , Get An Additional Error Error #1009?

I have created an invisible button myButton_btn inside a moviclip with instance name supermarket_mc. The supermarket_mc is based on a class Supermarket, which has the following code and works fine.

public class Supermarket extends MovieClip
{
public function Supermarket()[code]....

Now, if I have a button on Stage they work fine when I add a listener to them. However, when I add a listener to myButton_btn ie:

supermarket_mc.myButton_btn .addEventListener(MouseEvent.CLICK, turn);

myButton_btn does not react to click events and I get an additional error Error #1009: Cannot access a property or method of a null object reference.I suspect that the MouseEvent.CLICK in the class definition overrides the MouseEvent.CLICK on the nested button.