When trying to upload to chrome extensions, I get numerous errors saying Uncaught TypeError: Cannot set properties of null (setting 'innerHTML')

var startingMinutes = 30;
let time = startingMinutes * 60;

const countdownEl = document.getElementById('countdown');

setInterval(updateCountdown, 1000)

function updateCountdown() {
  countdownEl.innerHTML = `${minutes}:${seconds}`;
  const minutes = Math.floor(time / 60);
  let seconds = time % 60;
  seconds = seconds < 10 ? '0' + seconds : seconds;
  time--;
  time = time < 0 ? 0 : time;
  if (time == 0) {
    fn();
    time = startingMinutes * 60;
    return;
  }

  function fn() {
    alert("!")
  }
}