Slate arguments not in member

[BugSwat]

Using SNew for constructor with arguments? I created the following code:

  public:
	SLATE_BEGIN_ARGS(MyClass)
		: _XXX()
		{}
		SLATE_ATTRIBUTE(TSharedPtr<StyleBase>, XXX)
	SLATE_END_ARGS()

	void Construct(const FArguments& InArgs);

	TSharedPtr<StyleBase> XXX;

In MyClass::Construct I try to use it, but I have the following situation

XXX.IsValid() -> false
InArgs._XXX.Get().IsValid() -> true

I am doing something not or wrong. Any ideas?
Should not the member be set automatically?
Or do I have to set the member inside the Construct() method?