Quiz Application Console errors

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;
};

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 += "<h1> Your Score is: " + quiz.score + "</h2>";
    ``  this.populateIdWidthHTML("quiz", gameOverHTML);
    },

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

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

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"),
];

var quiz = new Quiz (questions);

QuizUI.displayNext();