Index: cat_event_handler.php
===================================================================
--- cat_event_handler.php	(revision 14239)
+++ cat_event_handler.php	(working copy)
@@ -955,9 +955,11 @@
 
 		$file_helper->LoadItemFiles($object);
 
-		// set item's additional categories to virtual field (used in editing)
-		$item_categories = $this->getItemCategories($object->GetDBField('ResourceId'));
-		$object->SetDBField('MoreCategories', $item_categories ? '|'.implode('|', $item_categories).'|' : '');
+		if ( array_key_exists('MoreCategories', $object->VirtualFields) ) {
+			// set item's additional categories to virtual field (used in editing)
+			$item_categories = $this->getItemCategories($object->GetDBField('ResourceId'));
+			$object->SetDBField('MoreCategories', $item_categories ? '|'.implode('|', $item_categories).'|' : '');
+		}
 	}
 
 	function OnAfterItemUpdate(&$event)