Error when catching objects and lives count wont decrease? - actionscript-3

Question

I'm currently making a catching game. So the user have to catch the right letter to get a score and go to the next question.
The letters are movieclips and I made the falling animation with motion tween.
I converted the avatar to a movieclip too.
When the avatar hits the right letter, the user will get 100 points and when the avatar hits the wrong letter, there will be a pop up button that says "try again" and it will lost its lives by 1.
Here's my code:
stop();
import flash.events.KeyboardEvent;
stage.addEventListener(KeyboardEvent.KEY_DOWN, moveavatar);
avatarmove.addEventListener(Event.ENTER_FRAME, handleCollision);
trybutton.visible = false;
var myscore = 0;
var mylives = 5;
score.text = myscore+"";
lives.text = mylives+"";
//Move avatar function
function moveavatar (event:KeyboardEvent):void
{
avatarmove.x = Math.max(0,Math.min(stage.stageWidth-avatarmove.width,avatarmove.x - int(event.keyCode==37) * 10 + int(event.keyCode==39) * 10));
}
//Collision between the avatar and the letters
function handleCollision(e:Event):void
{
if(avatarmove.hitTestObject(a_alphabet))
{
myscore+=100;
a_alphabet.visible = false;
gotoAndStop(2);
}
else if(avatarmove.hitTestObject(o_alphabet))
{
mylives-=1;
o_alphabet.visible = false;
trybutton.visible = true;
}
}
//try again button
trybutton.addEventListener (MouseEvent.CLICK, ayo1);
function ayo1(event:MouseEvent):void{
o_alphabet.visible = true;
trybutton.visible = false;
}
I keep getting this error when the avatar hits the letter:
TypeError: Error #2007: Parameter hitTestObject must be non-null.
at flash.display::DisplayObject/_hitTest()
at flash.display::DisplayObject/hitTestObject()
at gamecompletethewords_fla::MainTimeline/handleCollision()
And this error on the next frame:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at gamecompletethewords_fla::MainTimeline/handleCollision2()
And the the lives won't decrease when the avatar hits the wrong letter, but when I move to the next frame it will decrease by -27. It's just really weird.
Can someone help me with these problems? I'm totally new to this and I have no idea how to fix it :(