vs2017 UWP Flavored Project Won't Show XAML Designer

Hi All,

I have an extension project which normally flavors WPF projects. It's worked fine for ages. Now I want it to flavor a UWP project.

The idea is I have various managers in the extension that allow you to extend the new project (real time data acquisition setup and stuff like that). It all works fine.

But now when I come to extend the UWP and C# projects {A5A43C5B-DE2A-4C0C-9213-0A381AF9435A};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC} with my extension project GUID first, the XAML designer will not show even the simplest XAML Page. Not even an empty Grid.

I get a System.InvalidOperationException and the designer won't load, regardless of which option I choose. I get only the following information:

    at

    Microsoft.VisualStudio.DesignTools.SurfaceDesigner.DesignerService.CreateDesigner(

        IHostSourceItem item, IHostTextEditor editor, CancellationToken cancelToken)

    at

    Microsoft.VisualStudio.DesignTools.DesignerContract.IsolatedDesignerService.IsolatedDesignerView.CreateDesignerViewInfo(

        CancellationToken cancelToken)

Since debugging the Designer in vs2017 has changed, I'm a bit stuck! But remember, my underlying extension code has worked fine for a long time, and since there is no code-behind I can't see why the designer would be executing code that throws an exception. Maybe I haven't implemented a new interface for UWP support in the extension?

Any ideas, please???

Thanks, Loz.