Error 1009

Hello! I'm debugging my work, and later on, the "Error 1009" appears. I really don't know how to solve that error and tbh, I don't really understand the error that I've always encountered. So here's my code:

import flash.text.*;

import flash.display.MovieClip;

import flash.net.URLLoader;

import flash.net.URLRequest;

import flash.events.MouseEvent;

import flash.events.Event;

import flash.display.DisplayObject;

import flash.geom.Point;

import flash.display.Stage;

import flash.display.Sprite;

import flash.utils.Timer;

import flash.events.TimerEvent;

nohint.visible = false;

txtSkor.text = String(iskoru);

txtLives.text = String(buhay);

txtHints.text = String(hinto);

txtHint.visible = false;

var gamestage: int = 0;

var gameTimer:Timer = new Timer(1000);

gameTimer.addEventListener(TimerEvent.TIMER, countDown);

gameTimer.start();

function countDown(event: TimerEvent): void {

  txtSec.text = String(sec);

  sec--;

  if (sec < 0) {

  gameTimer.stop();

  gameTimer.removeEventListener(TimerEvent.TIMER, countDown);

  maliWin.visible = true;

  quizquest.visible = false;

  ans11.visible = false;

  ans12.visible = false;

  ans13.visible = false;

  btnHint.visible = false;

  tim.visible = false;

  }

  ans11.addEventListener(MouseEvent.CLICK, buttones1);

  ans12.addEventListener(MouseEvent.CLICK, buttones2);

  ans13.addEventListener(MouseEvent.CLICK, buttones3);

  function buttones1(event: MouseEvent): void {

  corekWin.visible = true;

  iskoru += 10;

  //txtSkor.text = String(iskoru);

  trace("click successful");

  }

  function buttones2(event: MouseEvent): void {

  maliWin.visible = true;

  buhay -= 1;

  //txtLives.text = String(buhay);

  trace("click successful");

  }

  function buttones3(event: MouseEvent): void {

  maliWin.visible = true;

  buhay -= 1;

  //txtLives.text = String(buhay);

  trace("click successful");

  }

  maliWin.btnGobak.addEventListener(MouseEvent.CLICK, bey);

  function bey(event: MouseEvent): void {

  maliWin.visible = false;

  }

  corekWin.btnGo.addEventListener(MouseEvent.CLICK, gura);

  function gura(event: MouseEvent): void {

  corekWin.visible = false;

  quizquest.visible = false;

  ans11.visible = false;

  ans12.visible = false;

  ans13.visible = false;

  btnHint.visible = false;

  txtSec.visible = false;

  tim.visible = false;

  playergril.visible = true;

  stage.addEventListener(Event.ENTER_FRAME, loofa);

  }

}

function loofa(event: Event): void {

  if (doorOpen == false) {

  if (playergril.hitTestObject(pinto)) {

  pinto.gotoAndStop(2);

  playergril.visible = false;

  doorOpen = true;

  trace("door open");

  gotoAndStop(77);

  }

  }

}

btnHint.addEventListener(MouseEvent.CLICK, wee);

function wee(event: MouseEvent): void {

  hinto -= 1;

  txtHint.visible = true;

  if (hinto == 0) {

  nohint.visible = true;

  }

}

Note: the bolded line is the main cause of it or pointed error.

And the error says:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

  at RECOVER_dmgamedemover_fla::MainTimeline/countDown()[RECOVER_dmgamedemover_fla.MainTimeline::frame76:27]

  at flash.utils::Timer/_timerDispatch()

  at flash.utils::Timer/tick()

When I look at the side (Debug Console) contains some variables and objects. The other dynamic texts have been declared (flash.text.TextField) while the txtSec, the another dynamic text, is null. I have no idea on what's going on it. What would be the best solution for it? Any suggestions or ideas are appreciated. Thanks!