You seem to have a deep misunderstanding of how the Spark SKinning Model should work. You should not reference the hostComponent from the skin in order to display items. You should create the element in the skin; and use the same name. So if the hostComponent has a skin part defined like this:
[Bindable]
[SkinPart]
public var titleBar:IVisualElement;
the skin should have something like this:
<s:DataGroup width="100%" id="titleBar" >
</s:DataGroup>
I suggest reading through this information on Spark Skinning, and also learn the Component LifeCycle.