RadDock's client-side object does not have get_parent() property. To get the id of the zone, which contains the dock you will have to use code like this:
var zoneID = this.get_DockZoneID();
if you need a reference to the zone's client-side object, use this:
var zone = $find(zoneID);
I hope this helps.
Valeri Hristov (Senior Developer, MCSD)
the Telerik team