Flash player error message

programming flash
1

I converted the Scrolling thumbnail panel tutorial at gotoAndLearn to AS3.
Here is the code I used to create the scrolling thumbnail panel:
//////////////////////////////////////////////////////////////////////////////

panel.addEventListener(MouseEvent.MOUSE_OVER, panelOver);

function panelOver(event:MouseEvent):void
{
panel.removeEventListener(MouseEvent.MOUSE_OVER, panelOver);
panel.addEventListener(Event.ENTER_FRAME, scrollPanel);

}

var b:Rectangle = stroke.getBounds(this);

function scrollPanel(event:Event):void

{
if(mouseX < b.left || mouseX > b.right || mouseY < b.top || mouseY > b.bottom) {
panel.removeEventListener(Event.ENTER_FRAME, scrollPanel);
panel.addEventListener(MouseEvent.MOUSE_OVER, panelOver);
}

if(panel.x > 200) {
panel.x = 200;
}
if(panel.x < -450) {
panel.x = -450;
}
var xdist = mouseX - 512;
panel.x += -(xdist / 7);
}

//////////////////////////////////////////////////////////////////////////////////////////////////////////

panel.addEventListener(MouseEvent.CLICK, btnClick);
var btnName = “”;
function btnClick(event:MouseEvent):void
{
btnName = event.target.name;
gotoAndStop(btnName);
}

Everything seems to be working fine but I get an
(TypeError: Error #1009: Cannot access a property or method of a null object reference.at NewShow_N03_C_fla::MainTimeline/scrollPanel()[NewShow_N03_C_fla.MainTimeline::frame15:36])
from the Player.
Please help.