Need help with code issues

I'm making a simple platformer game for a project, and have everything working fairly well, but when I added a section to respawn the character when they fall off the map, I get an error "TypeError: Error #1009:Cannot access a property or method of a null object reference. at DRAFTEVERYTHING_fla::MainTimeline/fl_MoveInDirectionOfKey()" and it makes the code repeat, causing the game to go in 2x speed, and it increases every time you fall off. My main code segment looks like this

var upPressed:Boolean = false;
var downPressed:Boolean = false;
var leftPressed:Boolean = false;
var rightPressed:Boolean = false;

var xSpeed:Number = 0;
var jumpMulti = 25;
var xSpeedMulti:Number = 4.5;
var xSpeedDiv:Number = .75;

var backPosX:Number = 0;
var backPosY:Number = 800;

var sideCheckHit:Boolean = false;
var leftRightSwitch:Boolean;

var ySpeed:Number = 0;
var gravityMulti:Number = 2;
var groundCheckHit:Boolean = false;

var death:Boolean = false;

player.addEventListener(Event.ENTER_FRAME, fl_MoveInDirectionOfKey);
stage.addEventListener(KeyboardEvent.KEY_DOWN, fl_SetKeyPressed);
stage.addEventListener(KeyboardEvent.KEY_UP, fl_UnsetKeyPressed);

function fl_MoveInDirectionOfKey(event:Event)
{
if (upPressed && groundCheckHit == true)
{
ySpeed -= jumpMulti;
}
if (downPressed)
{
background.y -= 5;
}
if (leftPressed)
{
xSpeed -= xSpeedMulti;
}
if (rightPressed)
{
xSpeed += xSpeedMulti;
}

groundCheck.x = player.x -10;
groundCheck.y = player.y +80;

xSpeed *= xSpeedDiv;

backPosX -= xSpeed;
backPosY -= ySpeed;

background.x = backPosX;
background.y = backPosY;

hall.x = backPosX * .2;
hall.y = backPosY * .2;

if(background.collision.hitTestPoint(groundCheck.x, groundCheck.y, true))
{
groundCheckHit = true;
}
else
{
groundCheckHit = false;
}
if(groundCheckHit)
{
ySpeed =- 0;
}
else
{
ySpeed += gravityMulti;
}

if (background.collision.hitTestPoint(player.x, player.y, true))
{
sideCheckHit = true;
}
else
{
sideCheckHit = false;
}
if(sideCheckHit)
{
if(leftRightSwitch)
{
backPosX -= 7;
xSpeed = 0;
}
else
{
backPosX +=7;
xSpeed = 0;
}
}

if(background.respawn.hitTestPoint(player.x, player.y, true))
{
death = true;
}
else
{
death = false;
}
if(death ==(true))
{
MovieClip(root).gotoAndStop(1, "Death Scene");
}
}

Any suggestions? I'm not very good with code.