function register(callback) {
    if (eventHandler !== null) {
      return;
    }
    eventHandler = createHandler(callback);
    window.addEventListener(eventType, eventHandler);

    return unregister;
  }
Registers an EventListener if it hasn't been registered.
@param {EventCallback} callback A function called after the event handler to handle its context.
@returns {unregister | void} A function to unregister the registered EventListener if registration is performed.