getOverlayById(id) {
    const overlay = this.overlayIdIndex_[id.toString()];
    return overlay !== undefined ? overlay : null;
  }
Get an overlay by its identifier (the value returned by overlay.getId()).
Note that the index treats string and numeric identifiers as the same. So
`map.getOverlayById(2)` will return an overlay with id `'2'` or `2`.
@param {string|number} id Overlay identifier.
@return {import("./Overlay.js").default|null} Overlay.
@api