My error looks like this :

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at QuizApp/createButtons()
at QuizApp()

My QuizApp is an Actionscript file that runs with another for my quiz.
The createButtons() is accessed like this

package{
public class QuizApp extends Sprite {
public function QuizApp() {
createQuestions();

It runs by itself fine but when i attatch it to play off a button i get this error, i would really appreciate some advice on how to go about figuring this out.
Thanks