Uncaught TypeError: Cannot read property 'text' of undefined

var QuizUI = {
  displayNext: function () { 
     if (quiz.hasEnded()) {  //checks if the quiz has ended
      this.displayScore();  //displays score
     } else {
        this.displayQuestion(); // gets the text from the current question and inserts into #question
        this.displayChoices();  //displays what question we are currently on
        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" + 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.currentQuizIndex + 1;
    this.populateIdWithHTML("progress","Question " + currentQuestionNumber + " of " + quiz.questions.length);
  },

}

// Create questions (stored in an array

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();

function Quiz(questions) { // takes an array of questions
  this.score = 0; //starts at zero
  this.questions = 0; //starts at 0
  this.currentQuizIndex = 0;
}

Quiz.prototype.guess = function(answer) { //takes in a string of answer, checks against current question
    if(this.getCurrentQuestion().isCorrectAnswer(answer)){ //if correct, add to score
      this.score++;
    }
    this.currentQuestionIndex++;  //move current index forward by 1
};

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

Quiz.prototype.hasEnded = function() {  //the quiz has ended, if the number is the same length or greater than, then the game is over
  return this.currentQuestionIndex >= this.questions.length;
};

function Question (text, choices, answer) { //constructor function
  this.text = text;
  this.choices = choices; //Array with 2 items, wrong answer and correct answer
  this.answer = answer;
}

Question.prototype.isCorrectAnswer = function (choice) { //method
  return this.answer === choice;
};