My quiz is not defined error message?

var QuizUI = {
    // Checks if the quiz has ended, if not displays the next question
    displayNext: function () {
        if ( capitalsQuiz.hasEnded() ) {
            this.displayScore();
        } else {
            this.displayQuestion();
            this.displayChoices();
            this.displayProgress();
        }
    },
    // Displays the question text
    displayQuestion: function () {
        this.populateIdWithHTML( "question", captialsQuiz.getCurrentQuestion().questions );
    },
    // Displays the answer choices
    displayChoices: function () {
        var choices = captialsQuiz.getCurrentQuestion().choices;
        // Loops through all the choices in the array
        for ( var i; i < choices.length; i++ ) {
            this.populateIdWithHTML("choice" + i, choices[1]);
            this.guessHandler("guess" + i, choices[1]);
        }
    },
    // Displays game over message with the users score
    displayScore: function () {
        var gameOverHTML = "<hl>Game Over</h1>";
        gameOverHTML += "<h2>Your score is " + captialsQuiz.score + "</h2>";
        this.populateIdWithHTML("captialsQuiz", gameOverHTML);
    },
    // A convenience method to grab the HTML placeholder and display text
    populateIdWithHTML: function (id, text) {
        var element = document.getElementById(id);
        element.innerHTML = text;
    },
    // Event handler to add score, move to the next question, and display it's the HTML
    guessHandler: function (id, guess) {
        var button = document.getElementById(id);
        button.onClick = function () {
            captialsQuiz.guess(guess);
            QuizUI.displayNext();
        }
    },
    // Displays the current question in the footer
    displayProgress: function () {
        var currentQuestionNumber = captialsQuiz.currentQuestionIndex + 1;
        this.populateIdWithHTML("progress", "Question" + currentQuestionNumber + " of " + captialsQuiz.guestions.length);
    }
};

// Create questions on country capitals
var question1 = new Question("What is the capital of England?", ["London", "Manchester"], "London");
var question2 = new Question("What is the capital of France?", ["Lyon", "Paris"], "Paris");

// Make a quiz out of an array of questions
var capitalsQuiz = new Quiz([question1, question2]);

// Display the quiz UI
QuizUI.displayNext();

/* 
A constructor to build a quiz object that contains an array of questions
You must add questions in the form of array
*/
function Quiz(questions) {
    this.score = 0;
    this.questions = questions;
    this.currentQuestionIndex = 0;
}

// A handy prototype to return the current question
Quiz.prototype.getCurrentQuestion = function () {
    return this.questions[this.currentQuestionIndex];
};

/*
A prototype to add a point to the user's score and
advance to the next question in the quiz array
*/
Quiz.prototype.guess = function (answer) {
    if ( this.getCurrentQuestion().isCorrectAnswer(answer) ) {
        this.score++;
    }
    this.currentQuestionIndex++;
};

Quiz.prototype.hasEnded = function () {
    if ( this.currentQuestionIndex >= this.questions.length ) {
        return true;
    } else {
        return false;
    }
}