Uncaught TypeError: this.board.drawHTMLBoard is not a function

/** 
 * Listens for click on `#begin-game` and calls startGame() on game object
 */

const game = new Game();

document.getElementById('begin-game').addEventListener('click', function() {
    this.style.display = 'none';
    document.getElementById('play-area').style.opacity = '1';

    game.startGame();
});

class Game {
    constructor() {
        this.board = new Board();
        this.players = this.createPlayers();
        this.ready = false;
    }


    /** 
     * Returns active player.
     * @return  {Object}    player - The active player.
     */
    get activePlayer() {
        return this.players.find(player => player.active);
    }


    /** 
     * Creates two player objects
     * @return  {array}    An array of two player objects.
     */
    createPlayers() {
        const players = [new Player('Player 1', 1, '#e15258', true),
                         new Player('Player 2', 2, '#e59a13')];
        return players;
    }


    /** 
     * Initializes game. 
     */
    startGame(){
        this.board.drawHTMLBoard();
        this.activePlayer.activeToken.drawHTMLToken();
        this.ready = true;
    }
}

class Board {
    constructor() {
        this.rows = 6;
        this.columns = 7;
        this.spaces = this.createSpaces();
    }

    /** 
     * Generates 2D array of spaces. 
     * @return  {array}     An array of space objects
     */
    createSpaces() {
        const spaces = [];

        for (let x = 0; x < this.columns; x++) {
            const col = [];

            for (let y = 0; y < this.rows; y++) {
                const space = new Space(x, y);
                col.push(space);
            }

            spaces.push(col);
        }

        return spaces;
    }

    /** 
     * Draws associated SVG spaces for all game spaces.
     */
    drawHTMLBoard() {
        for (let column of this.spaces) {
            for (let space of column) {
                space.drawSVGSpace();
            }
        }
    } 
}

class Space {
    constructor ( x, y ) {
        this.x = x;
        this.y = y;
        this.id = `space${x}-${y}`;
        this.token = null;
        this.diameter = 76;
        this.radius = this.diameter/2;
    }

    drawSVGSpace() {
        const svgSpace = document.createElementNS("http://www.w3.org/2000/svg", "circle");

        svgSpace.setAttributeNS(null, "id", this.id);
        svgSpace.setAttributeNS(null, "cx", (this.x * this.diameter) + this.radius);
        svgSpace.setAttributeNS(null, "cy", (this.y * this.diameter) + this.radius);
        svgSpace.setAttributeNS(null, "r", this.radius - 8);
        svgSpace.setAttributeNS(null, "fill", "black");
        svgSpace.setAttributeNS(null, "stroke", "none");

        document.getElementById("mask").appendChild(svgSpace);  
    }
}