Weird error in console

What does this error mean

Here’s the code:

let octave = 4;

const piano = Synth.createPiano();
const keys = [
  "C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"
];
let pianoDiv = null;
let removeKey = null;

const _HELPER = {
  playNextOctave: function(listElem) {
    piano.play("C", octave + 1, 1);
    listElem.stopPropagation();
  },
  initializeNextOctaveKey: function() {
    const append = document.createElement("div");
    append.className = "piano-white";
    append.dataset.key = "C-up";
    append.id = "removeIfOctaveIs8";
    append.addEventListener("pointerdown", _HELPER.playNextOctave);
    return append;
  },
  getNumberOfElementNodes: function(parent) {
    let numberOfChildren = 0;
    const NODE_ELEMENT_TYPE = 1;
    for (let i = 0; i < parent.childNodes.length; i++) {
      if (parent.childNodes[i].nodeType === NODE_ELEMENT_TYPE) {
        numberOfChildren++;
      }
    }
    return numberOfChildren;
  }
};

window.addEventListener("DOMContentLoaded", () => {
  pianoDiv = document.getElementById("piano");
  removeKey = document.getElementById("removeIfOctaveIs8");
  
  keys.forEach(e => {
    const list = document.querySelectorAll(`[data-key="${e}"]`);
    for (let i = 0; i < list.length; i++) {
      list[i].addEventListener("pointerdown", function(listElem) {
        piano.play(e, octave, 1);
        listElem.stopPropagation();
      });
    }
  });
  document.querySelector('[data-key="C-up"]').addEventListener("pointerdown", _HELPER.playNextOctave);
  document.getElementById("go-up").addEventListener("click", () => {
    if (octave >= 7) return;
    octave++;
    if (octave === 7) {
      pianoDiv.removeChild(removeKey);
    }
  });
  document.getElementById("go-down").addEventListener("click", () => {
    if (octave <= 2) return;
    octave--;
    
    if (_HELPER.getNumberOfElementNodes(pianoDiv) === 8) return;
    
    removeKey = _HELPER.initializeNextOctaveKey();
    pianoDiv.appendChild(removeKey);
  })
});

I don’t see any mention of Promises in my code, so why am I getting an error from a Promise?