AS3 addEventListener to dynamically named MC

Hello all,

I’m building a variable matching game. The player clicks one of four choices and then clicks the answer area. Their choice appears in the area, and if the answer is correct the answer container shifts down, highlighting the next answer area to be filled. Inside the container are a large number of these answer areas (60).

What I am trying to do is have only one be answerable at a time, the highlighted one, so that players can’t skip ahead despite seeing the empty spaces left to answer in. What I have right now isn’t working, and I could use some outside insight.

var i:int =1;

function sampleFunction(evt:MouseEvent):void{
    if(selection == e.currentTarget.targetCorrect){//targetCorrect is a string set statically
        e.currentTarget.troo = true;//troo is a Boolean variable inside each answer area
function mcAdvance():void{
    if(i == 61){
    else{, .15, {y: "+50"});
    MovieClip(this[""+i]).addEventListener(MouseEvent.CLICK, sampleFunction);

The error that I currently get is:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at MainTimeline/mcAdvance()
at MainTimeline/sampleFunction()

I realize I could use a for loop to add the event listener, but the project really calls for only one answerable area at a time. Any help would be greatly appreciated, this is the core of the last project section.