querySelector(query) {
    query = query.toUpperCase(); // @ts-ignore

    const iterator = new YXmlTreeWalker(this, element => element.nodeName && element.nodeName.toUpperCase() === query);
    const next = iterator.next();

    if (next.done) {
      return null;
    } else {
      return next.value;
    }
  }
Returns the first YXmlElement that matches the query.
Similar to DOM's {@link querySelector}.

Query support:
  - tagname
TODO:
  - id
  - attribute

@param {CSS_Selector} query The query on the children.
@return {YXmlElement|YXmlText|YXmlHook|null} The first element that matches the query or null.

@public