XWIKI-21616: Potential NPE when having ToggleableFilterPreferenceClass xobjects with holes

  * Protect against NPE and cover it in unit test

xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-filters/xwiki-platform-notifications-filters-default/src/main/java/org/xwiki/notifications/filters/internal/UserAddedEventListener.java

@@ -134,7 +134,9 @@ private void copyToggleableFilterPreferences(XWikiDocument userDocument) throws
                 List<BaseObject> xObjects = notificationConfiguration.getXObjects(TOGGLEABLE_FILTER_PREFERENCE_CLASS);
 
                 for (BaseObject xObject : xObjects) {
-                    userDocument.addXObject(xObject.duplicate());
+                    if (xObject != null) {
+                        userDocument.addXObject(xObject.duplicate());
+                    }
                 }
             }
         } finally {

xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-filters/xwiki-platform-notifications-filters-default/src/test/java/org/xwiki/notifications/filters/internal/UserAddedEventListenerTest.java

@@ -129,7 +129,7 @@ void onEvent() throws NotificationException, XWikiException
         when(obj1.duplicate()).thenReturn(obj1bis);
         when(obj2.duplicate()).thenReturn(obj2bis);
         when(wikiNotificationConfiguration.getXObjects(UserAddedEventListener.TOGGLEABLE_FILTER_PREFERENCE_CLASS))
-            .thenReturn(Arrays.asList(obj1, obj2));
+            .thenReturn(Arrays.asList(obj1, null, obj2, null));
 
         this.userAddedEventListener.onEvent(null, userDoc, null);
         verify(this.notificationFilterPreferenceManager).saveFilterPreferences(documentReference, expectedSet);