Where is the error in my code? as3 - actionscript-3

Question

I'm trying to make a simple quiz game but my code started passing weird things to the questionsbox textbox. The code was running fine the first time I tried it. After a while, questionbox started displaying random characters like a, i, o, r and e when I try to run the test. Also, inputbox also seems to only accept these characters now.
import flash.events.MouseEvent;
import flash.text.TextField;
import flash.text.TextFormat;
import flash.events.Event;
stop();
start_btn.addEventListener(MouseEvent.CLICK, startQuiz)
function startQuiz(event:MouseEvent):void {
nextFrame();
var questions:Array = new Array;
var answers:Array = new Array;
var userAnswers:Array = new Array;
var grade:String
var score:int = 0;
var quizProgress:int = 0;
var myTextField = new TextField();
// Edit questions here
questions[0] = "What is x if x - 5 = 3?";
questions[1] = "What is the square root of 49?";
questions[2] = "What is the area of a circle whose diameter is 4?";
questions[3] = "What is 5 + 10?";
questions[4] = "What is -15 * 2?";
// Edit answers here
answers[0] = "8";
answers[1] = "7";
answers[2] = "12.57";
answers[3] = "15";
answers[4] = "-30";
generateQuestion();
function generateQuestion()
{
questionbox.text = questions[quizProgress];
}
function updateScore()
{
for(var i:int = 0; i < questions.length; i++)
{
if(userAnswers[i] == answers[i])
{
score++;
}
}
}
function generateGrade()
{
switch (score)
{
case 0:
grade = "F";
break;
case 1:
grade = "D";
break;
case 2:
grade = "C";
break;
case 4:
grade = "B";
break;
case 5:
grade = "A";
break;
}
}
submit_btn.addEventListener(MouseEvent.CLICK, submitAnswer)
function submitAnswer(event:Event):void
{
if (quizProgress < 4)
{
userAnswers[quizProgress] = inputbox.text;
quizProgress++;
generateQuestion();
inputbox.text = "";
}
else
{
userAnswers[quizProgress] = inputbox.text;
quizProgress++
inputbox.text = "";
questionbox.text = "";
updateScore();
generateGrade();
submit_btn.removeEventListener(MouseEvent.CLICK, submitAnswer)
nextFrame();
grade_text.text = grade;
}
}
}