setCaption (caption) {
    let captionElement = this._activeElement.querySelectorAll('.css_modal--gallery_caption')[0];

    if (!captionElement) {
      return;
    }

    captionElement.innerHTML = '';

    if (caption) {
      captionElement.innerHTML = '<p>' + caption + '</p>';
    }
  }
Set a caption for a given modal
@param {String} caption Caption to insert