Actionscript 3: mouse_over & mouse_out causing Error #1009? - actionscript-3

Question

When I mouse over the buttonSprite, it adds displaySprite to the stage, and when I mouse out buttonSprite, the displaySprite is removed.
My problem is that when I swiftly mouse over and out the buttonSprite several times, the displaySprite is not be removed and an error message(Error #1009) is displayed. Even I type "displaySprite = null", it's still not working. Any suggestions? Thanks
var buttonSprite:Sprite = new Sprite();
addChild(buttonSprite);
buttonSprite.addEventListener(MouseEvent.MOUSE_OVER, overSprite);
var displaySprite:Sprite;
function overSprite(e:MouseEvent):void{
displaySprite = new Sprite();
addChild(displaySprite);
buttonSprite.addEventListener(MouseEvent.MOUSE_OUT, outSprite);
}
function outSprite(e:MouseEvent):void{
removeChild(displaySprite);
displaySprite = null;
}

Answer 1

There is no guarantee that the events will fire in order.
In your case you do not have to instantiate the displaySprite multiple times.
Just don't null it out and the object will be there.
var buttonSprite:Sprite = new Sprite();
addChild(buttonSprite);
buttonSprite.addEventListener(MouseEvent.MOUSE_OVER, overSprite);
//you only need to create it once.
var displaySprite:Sprite = new Sprite();
function overSprite(e:MouseEvent):void{
addChild(displaySprite);
buttonSprite.addEventListener(MouseEvent.MOUSE_OUT, outSprite);
}
function outSprite(e:MouseEvent):void{
removeChild(displaySprite);
}

Answer 2

How about this?
var buttonSprite:Sprite = new Sprite();
addChild(buttonSprite);
buttonSprite.addEventListener(MouseEvent.MOUSE_OVER, overSprite);
buttonSprite.addEventListener(MouseEvent.MOUSE_OUT, outSprite);
var displaySprite:Sprite;
addChild(displaySprite);
displaySprite.visible=false;
function overSprite(e:MouseEvent):void
{
displaySprite.visible = true;
}
function outSprite(e:MouseEvent):void
{
displaySprite.visible = false;
}

Answer 3

The problem is MouseEvent.MOUSE_OVER is dispatched multiple times when you mouse over a display object. What you should try is MouseEvent.ROLL_OVER and MouseEvent.ROLL_OUT those two events are dispatched once.
Something along the lines of :
var buttonSprite:Sprite = new Sprite();
addChild(buttonSprite);
buttonSprite.addEventListener(MouseEvent.ROLL_OVER, overSprite);
buttonSprite.addEventListener(MouseEvent.ROLL_OUT, outSprite);
var displaySprite:Sprite;
function overSprite(e:MouseEvent):void
{
if(!displaySprite)
{
displaySprite = new Sprite();
addChild(displaySprite);
}
}
function outSprite(e:MouseEvent):void{
if(displaySprite)
{
removeChild(displaySprite);
displaySprite = null;
}
}