The Quiz UI isn't displaying everything.

//Create Questions
var questions = [
    new Question("Who was the first President of the United States?", [ "George Washington", "Thomas Jefferson" ], "George Washington"),
    new Question("What is the answer to the Ultimate Question of Life, the Universe, and Everything?", ["Pi","42"], "42")
];

//Create Quiz
var quiz = new Quiz(questions);

//Display Quiz
QuizUI.displayNext();

var QuizUI = {
    displayNext: function() {
        if (quiz.hasEnded()) {
            this.displayScore();
        } else {
            this.displayQuestion();
            this.displayChoices();
            this.displayProgress();
    }
},
displayQuestion: function() {
    this.populateIdWithHTML("question", quiz.getCurrentQuestion().text);
},
displayChoices: function() {
    var choices = quiz.getCurrentQuestion().choices;

    for(var i = 0; i < choices.length; i++) {
        this.populateIdWithHTML("choice" + i, choices[i]);
        this.guessHandler("guess" + i, choices[i]);
    }
},

displayScore: function() {
    var gameOverHTML = "<h1>Game Over</h1>";
    gameOverHTML += "<h2> Your score is: " + quiz.score + "</h2>";
    this.populateIdWithHTML("quiz", gameOverHTML);
},

populateIdWithHTML: function(id, text) {
    var element = document.getElementById(id);
    element.innerHTML = text;
},
guessHandler: function(id, guess) {
    var button = document.getElementById(id);
    button.onclick = function() {
        quiz.guess(guess);
        QuizUI.displayNext();
    }
},

displayProgress: function() {
    var currentQuestionNumber = quiz.currentQuestionIndex + 1;
    this.populateIdWithHTML("progress", "Question" + currentQuestionNumber + " of " + quiz.questions.length);
    }
};

function Quiz(questions) {
    this.score = 0;
    this.questions = questions;
    this.currentQuestionIndex = 0;
}

Quiz.prototype.guess = function(answer) {
    if(this.getCurrentQuestion().isCorrectAnswer(answer)) {
        this.score++;
    }
    this.currentQuestionIndex++;
};

Quiz.prototype.getCurrentQuestion = function() {
    return this.questions[this.currentQuestionIndex];
};

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