TypeError: Error 1009 in Adobe Animate CC AS3

Hello again! I'm creating an assessment quiz for my game. It works fine (just the partial app) but, when I proceed to the next frame which is the assessment quiz, it didn't work. Suddenly, the run-time error appears:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
  at PropoCon/frame11()[PropoCon::frame11:35]
  at flash.display::MovieClip/gotoAndStop()
  at PropoCon/sunod9()[PropoCon::frame10:6]

Here's my code from frame 11:

import flash.text.TextField;

var qno = 0;
var rnd1;
var rnd2;
tick.visible = false;
cross.visible = false;
korekokWin.visible = false;
var right_answers = 0;
var wrong_answers = 0;

addChild(korekokWin);

function change_question() {
  korekokWin.visible = false;
   if (tick.visible) {
  right_answers++;
   }
   if (cross.visible) {
  wrong_answers++;
   }
   if (qno == questions.length) {
  gotoAndPlay(11);
   }
   else {
  tick.visible = false;
  cross.visible = false;
  rnd1 = Math.ceil(Math.random() * 3);
  rnd2 = Math.ceil(Math.random() * questions.length) - 1;
  tanong.text = questions[rnd2];
   if (questions[rnd2] == "x") {
  change_question();
   }
  questions[rnd2] = "x";
  enable_disable(1);//this is the line 35
   if (rnd1 == 1) {
  opt1.text = answers[rnd2][0];
  opt2.text = answers[rnd2][1];
  opt3.text = answers[rnd2][2];
   }
   if (rnd1 == 2) {
  opt1.text = answers[rnd2][2];
  opt2.text = answers[rnd2][0];
  opt3.text = answers[rnd2][1];
   }
   if (rnd1 == 3) {
  opt1.text = answers[rnd2][1];
  opt2.text = answers[rnd2][2];
  opt3.text = answers[rnd2][0];
   }
   }
}

function enable_disable(a) {
   if (a == 0) {
  shade1.mouseEnabled = false;
  shade2.mouseEnabled = false;
  shade3.mouseEnabled = false;
   }
   if (a == 1) {
  shade1.mouseEnabled = true;
  shade2.mouseEnabled = true;
  shade3.mouseEnabled = true;
   }
}

change_question();

btnNek.addEventListener(MouseEvent.CLICK, ButtonAction1);
function ButtonAction1(eventObject: MouseEvent😞void {
  qno++;
  change_question();
}

shade1.addEventListener(MouseEvent.CLICK, ButtonAction2);
shade2.addEventListener(MouseEvent.CLICK, ButtonAction3);
shade3.addEventListener(MouseEvent.CLICK, ButtonAction4);

function ButtonAction2(eventObject: MouseEvent😞void {
  enable_disable(0);
   if (rnd1 == 1) {
  tick.visible = true;
  tick.y = shade1.y;
  korekokWin.visible = false;
   }
   else {
  cross.visible = true;
  cross.y = shade1.y;
  korekokWin.visible = true;
  korekokWin.koreksyon.text = korek[rnd2];
   }
}
function ButtonAction3(eventObject: MouseEvent😞void {
  enable_disable(0);
   if (rnd1 == 2) {
  tick.visible = true;
  tick.y = shade2.y;
  korekokWin.visible = false;
   }
   else {
  cross.visible = true;
  cross.y = shade2.y;
  korekokWin.visible = true;
  korekokWin.koreksyon.text = korek[rnd2];
   }
}
function ButtonAction4(eventObject: MouseEvent😞void {
  enable_disable(0);
   if (rnd1 == 3) {
  tick.visible = true;
  tick.y = shade3.y;
  korekokWin.visible = false;
   }
   else {
  cross.visible = true;
  cross.y = shade3.y;
  korekokWin.visible = true;
  korekokWin.koreksyon.text = korek[rnd2];
   }
}

And this is from frame 10:

stop();
next.addEventListener(MouseEvent.CLICK, sunod9);
backward.addEventListener(MouseEvent.CLICK, balik8);

function sunod9(event:MouseEvent😞void{
  gotoAndStop(11);//this is in the line 6
}

function balik8(event:MouseEvent😞void{
  gotoAndStop(9);
}

Both codes are from movieclip timeline. My codes seem right but I dunno what would be the cause of problem. Is there something wrong in my code? Did you think that my code is something incomplete or what? What do you think is the main cause of those problems? Any suggestions or help will be appreciated. Thanks!