getSingletonElementInstance = (tag, parentElement = document.body, createEl) => {
    let el = document.querySelector(tag);
    if (el) {
      return el;
    }
    el = createEl ? createEl() : document.createElement(tag);
    return parentElement.insertBefore(el, parentElement.firstChild);
  }
Returns a singleton HTML element, inserted in given parent element of HTML page,
used mostly to store and share global resources between multiple UI5 Web Components runtimes.

@param { string } tag the element tag/selector
@param { HTMLElement } parentElement the parent element to insert the singleton element instance
@param { Function } createEl a factory function for the element instantiation, by default document.createElement is used
@returns { Element }