fixed null error with notif grouping

src/notifications.js

@@ -344,6 +344,10 @@ var async = require('async'),
 
 		notifications = mergeIds.reduce(function(notifications, mergeId) {
 			isolated = notifications.filter(function(notifObj) {
+				if (!notifObj.hasOwnProperty('mergeId')) {
+					return false;
+				}
+
 				return notifObj.mergeId.split('|')[0] === mergeId;
 			});