Fix excludeSiteTreeClassNames (#64)

src/Model/Lumberjack.php

@@ -100,21 +100,25 @@ public function stageChildren($showAll = false)
         }
 
         $this->owner->extend("augmentStageChildren", $staged, $showAll);
-        $this->excludeSiteTreeClassNames($staged);
+        $staged = $this->excludeSiteTreeClassNames($staged);
         return $staged;
     }
 
     /**
-     * Excludes any hidden owner subclasses
+     * Excludes any hidden owner subclasses. Note that the returned DataList will be a different
+     * instance from the original.
+     *
      * @param DataList $list
+     * @return DataList
      */
     protected function excludeSiteTreeClassNames($list)
     {
         $classNames = $this->owner->getExcludedSiteTreeClassNames();
         if ($this->shouldFilter() && count($classNames)) {
             // Filter the SiteTree
-            $list->exclude('ClassName', $classNames);
+            $list = $list->exclude('ClassName', $classNames);
         }
+        return $list;
     }
 
     /**
@@ -156,7 +160,7 @@ public function liveChildren($showAll = false, $onlyDeletedFromStage = false)
         if (!$showAll && DataObject::getSchema()->fieldSpec($this->owner, 'ShowInMenus')) {
             $children = $children->filter('ShowInMenus', 1);
         }
-        $this->excludeSiteTreeClassNames($children);
+        $children = $this->excludeSiteTreeClassNames($children);
 
         return $children;
     }