Null reference issue

I have this fixed, but I’m just trying to understand why it does what it does. I have a library movieClip containing 10 frames. Each frame is a different question and its answers, which are multiple choice. To handle the answer clicks I have four transparent shapes covering them - qa, qb, qc, and qd.
In the associated class I do something like:

qa.addEventListener(MouseEvent.CLICK, qaClicked);

and three more for qb, qc, qd.

All works fine. Except… in frame 6 I have a true / false question, so just qa and qb. So I deleted the qc and qd shapes. This is the only frame / question where there are not four answers.

When I go to question 7 (or any question after 6) then and try adding the four event listeners I get a null object reference for qc, and qd. Even though they are there…

To solve, I just placed qc and qd back in frame 6…

But I’m wondering why I needed to.

Thanks, hope it makes sense.