Moving my Node.js app to Express, getting Document Null errors

I have my fully working app here that I’m trying to host on a localhost server using Express.js

https://jsfiddle.net/mostafa91/6axesy2t/2/

Here is my server.js that I run with “node server.js”:

const express = require('express');
const app = express();
const port = 2525;

app.get('/', (req, res) => {
    res.sendFile(__dirname + '/public/Gameboard.html');
});

app.listen(port, () => {
    console.log(`Tic-Tac-Toe listening at http://localhost:${port}`);
});

app.use(express.static('public'));

I keep getting errors when I try to play the game (vs. Players option):

Uncaught TypeError: Cannot read properties of null (reading 'classList')
    at setBoardHoverClass (Gameboard.js:74)
    at startGame (Gameboard.js:32)
    at Gameboard.js:20

I’ve included my file structure as well: Here