event trigger error

Hi, i would like to add a trigger on a button, but i have a null referenc exception. i tried to be as close as possible to this example but it doesn't work : http://answers.unity3d.com/questions/854251/how-do-you-add-an-ui-eventtrigger-by-script.html here is my code

 lcurrentButton.AddComponent<EventTrigger>();
 EventTrigger trigger = lcurrentButton.GetComponent<EventTrigger>();
 EventTrigger.Entry entry = new EventTrigger.Entry();
 entry.eventID = EventTriggerType.Drag;
 entry.callback.AddListener( (eventData) => { Swipe_mouse(); } );
 trigger.delegates.Add(entry);

the swipe_mouse contains just a debug log for now:

 public void Swipe_mouse()
 {
     print ("swipe_start");
 }

here is the error i have on the console: NullReferenceException: Object reference not set to an instance of an object

i don't know what the error is, becaus the trigger is created and i can even see it on the inspector.

Thanks