$cssFile must allow NULL

Problem/Motivation

After update from 2.1.2 to 2.1.4, I am having this error in my website:

TypeError: Cannot assign null to property Drupal\ajax_loader\ThrobberPluginBase::$cssFile of type string in Drupal\ajax_loader\ThrobberPluginBase->__construct() (line 60 of modules/contrib/ajax_loader/src/ThrobberPluginBase.php).

The property cssFile only support string now, but it can accept a null value. According to ajax_loade.module

 if ($css_file = $throbber->getCssFile()) {
   $libraries['ajax_loader.throbber']['css']['theme'][$css_file] = [];
 }

Proposed resolution

Accept NULL when the property is defined

protected ?string $cssFile;