refs #25122 fix button for activating / deactivating result grouping

* remove static from invoke method of GroupingCheckboxFactory

src/AjaxHandler/GroupingCheckboxFactory.php

@@ -35,7 +35,7 @@ class GroupingCheckboxFactory
      * @param ContainerInterface $container
      * @return \VuFindResultsGrouping\AjaxHandler\GroupingCheckbox
      */
-    public static function __invoke(ContainerInterface $container)
+    public function __invoke(ContainerInterface $container)
     {
         return new GroupingCheckbox(
             $container->get('VuFindResultsGrouping\Config\Grouping')