p5.js Cannot convert undefined or null to object

I am trying to make a grid for a game, and I set up the grid, but when I use a for loop to display all of the cells it gives me an error - Uncaught TypeError: Cannot convert undefined or null to object. I think that there is something undefined in the grid array but there shouldn't be. Another theory is that the grid is empty, although it should have 16 cells in it.

let bgCol = '#A6A6A6';
let rows = 4;
let cols = 4;
let grid = [];
let size = 50;


function setup() {
    createCanvas(windowWidth, windowHeight);
    background(bgCol);
    for (let r = 0; r < rows; r++) {
        for (let c = 0; c < cols; c++) {
            grid.push(new Cell({x: r * size, y: c * size}, 0));
        }
    }
}

function draw() {
    background(bgCol);
    for (let g of grid) {
        grid[g].dis();
    }
}

class Cell {
    constructor(pos, num, col = '#FFFFFF') {
        this.pos = pos;
        this.num = num;
        this.col = col; // White as placeholder until later
    }
    dis() {
        stroke('#000000');
        strokeWeight(2);
        fill(this.col);
        square(this.pos.x, this.pos.y, size, 5);
        if (this.num) {
            textSize(size - 5);
            text(this.num, this.pos.x + 5, this.pos.y + 5);
        }
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.11.0/p5.js"></script>

Solution

for...of iterates over values, not indexes. for...in, on the other hand, iterates over indexes.

In your case, simply change the line (grid[g].dis();) to g.dis();:

let bgCol = '#A6A6A6';
let rows = 4;
let cols = 4;
let grid = [];
let size = 50;


function setup() {
    createCanvas(windowWidth, windowHeight);
    background(bgCol);
    for (let r = 0; r < rows; r++) {
        for (let c = 0; c < cols; c++) {
            grid.push(new Cell({x: r * size, y: c * size}, 0));
        }
    }
}

function draw() {
    background(bgCol);
    for (let g of grid) {
        g.dis();
    }
}

class Cell {
    constructor(pos, num, col = '#FFFFFF') {
        this.pos = pos;
        this.num = num;
        this.col = col; // White as placeholder until later
    }
    dis() {
        stroke('#000000');
        strokeWeight(2);
        fill(this.col);
        square(this.pos.x, this.pos.y, size, 5);
        if (this.num) {
            textSize(size - 5);
            text(this.num, this.pos.x + 5, this.pos.y + 5);
        }
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.11.0/p5.js"></script>

As @ggorlen pointed out in a comment, better naming will help avoid future misunderstandings: change g to cell and dis to display.