fix js error when dragging between calendars
fix js error when dragging between calendars

oleg-babintsev/fullcalendar,fullcalendar/fullcalendar,oleg-babintsev/fullcalendar,fullcalendar/fullcalendar,arshaw/fullcalendar,arshaw/fullcalendar,fullcalendar/fullcalendar,fullcalendar/fullcalendar,arshaw/fullcalendar,arshaw/fullcalendar,oleg-babintsev/fullcalendar