Please report bugs here. If you are unsure whether something is a bug or an expected behavior, please post it on the "Technical Support" forum instead, and wait for a moderator to handle/move the post.
Thanks for your message. This is a known limitation of the current version of the designer.
As written on the Limitations page, "the XAML Editor is unable to preview custom types and user controls. Only native XAML types can be previewed in the XAML Editor at the moment."