ActionScript 3.0 :: Cannot Manage To The FocusSkin For A Datefield

I cannot manage to the focusSkin for a datefield.

In my css I have done the following:

mx|DateField { focus-skin: ClassReference("skins.myFocusSkin"); skinClass: ClassReference(".myInputSkin"); }

My focus Skin is as such:

import spark.components.supportClasses.SkinnableComponent ;
private var _target:SkinnableComponent;
public function get target():SkinnableComponent

[Code].....

However, when the datefield gets focus, I get the following error :

TypeError: Error #1009: Cannot access a property or method of a null object reference. at skins::myFocusSkin/updateDisplayList()