"TypeError: Error #1009: Cannot access a property or method of a null object reference" while using gotoAndplay function - actionscript-3

Question

I was trying to add a gameover screen with a restart button for my game.I had placed the restart button at frame 22.When my player dies it goes to frame 22 and i'm able to restart the game on clicking the button,but this message gets looped in the output area.Please help me how i can correct this issue.
Issue is not there when i remove the line
gotoAndPlay(22);
at Frame 17,but without that i will not get the desired functionality.
Please find my code below
At Frame 17 - Game code
stop();
import flash.events.Event;
import flash.events.MouseEvent;
var mouseIsDown = false;
var speed = 0;
var score = 0;
addEventListener(Event.ENTER_FRAME,mainLoop);
stage.addEventListener(MouseEvent.MOUSE_DOWN,clicked);
stage.addEventListener(MouseEvent.MOUSE_UP,unclicked);
function clicked(m:MouseEvent)
{
mouseIsDown = true;
}
function unclicked(m:MouseEvent)
{
mouseIsDown = false;
}
function mainLoop(e:Event)
{
score = score + 10;
output.text = "Score: "+score;
if(mouseIsDown)
{
speed -= 2;
}
else
{
speed+=2;
}
if(speed > 10) speed = 10;
if(speed < -10) speed = -10;
player.y += speed;
for(var i = 0; i < numChildren; i++)
{
if (getChildAt(i) is Cloud || getChildAt(i) is Boundary)
{
var b = getChildAt(i) as MovieClip;
if(b.hitTestObject(player))
{
for(var counter = 0; counter < 12; counter++)
{
var boom = new Boom();
boom.x = player.x;
boom.y = player.y;
boom.rotation = Math.random() * 360;
boom.scaleX = boom.scaleY = 0.5 + Math.random();
addChild(boom);
}
player.visible = false;
removeEventListener(Event.ENTER_FRAME,mainLoop);
gotoAndPlay(22);
}
}
}
}
At frame 22 - Restart screen
stop();
import flash.events.MouseEvent;
foutput.text = "Score: "+ fscore;
btn_playagain.addEventListener(MouseEvent.CLICK, playagain);
function playagain(m:MouseEvent)
{
gotoAndPlay(17);
}
btnback3.addEventListener(MouseEvent.CLICK, backMain3);
function backMain3(m:MouseEvent)
{
gotoAndPlay(1);
}
At frame 1 - Main Menu screen
stop();
import flash.events.MouseEvent;
import flash.system.fscommand;
btnnewgame.addEventListener(MouseEvent.CLICK, newGame);
function newGame(m:MouseEvent)
{
gotoAndPlay(17);
}
btnins.addEventListener(MouseEvent.CLICK, instruct);
function instruct(m:MouseEvent)
{
gotoAndPlay(6);
}
btncredits.addEventListener(MouseEvent.CLICK, credits);
function credits(m:MouseEvent)
{
gotoAndPlay(11);
}
btnexit.addEventListener(MouseEvent.CLICK, exitfunc);
function exitfunc(m:MouseEvent)
{
fscommand("quit");
}
At Frame 6 - Instructions Screen
stop();
btnback1.addEventListener(MouseEvent.CLICK, backMain1);
function backMain1(m:MouseEvent)
{
gotoAndPlay(1);
}
At Frame 11 - Credits Screen
stop();
btnback2.addEventListener(MouseEvent.CLICK, backMain2);
function backMain2(m:MouseEvent)
{
gotoAndPlay(1);
}

Answer 1

That error means that you are trying to call a method on a null object, meaning one of the objects you are using on frame 22 doesn't actually exist at that moment.
The likely candidates for the offending variable are foutput, btn_playagain, and btnback3. Check to make sure that they are on the stage at frame 22, and are spelt correctly.
You use output.text on frame 17, are you sure that it should be foutput.text on frame 22?