Fix minor bug when option is not defined (#2486)

* Fix redraw order

* Fix error when option is not defined

lib/timeline/Timeline.js

@@ -70,7 +70,8 @@ function Timeline (container, items, groups, options) {
   } else {
     this.options.rtl = options.rtl;
   } 
-  this.options.rollingMode = options.rollingMode; 
+
+  this.options.rollingMode = options && options.rollingMode; 
 
   // all components listed here will be repainted automatically
   this.components = [];