Is this a bug of 10.14 Beta 4?

I'm trying to evaluate MacOS 10.14 beta 4 with our Xamarin app.

We found close button disappear on your sample at https://github.com/xamarin/mac-samples/tree/master/yosemite/VisualEffectPlayground as attached screen shot.

Is this OS bug ?
Or
Should we have some modification to support 10.14 with Xamarin ?

And, Visual studio for Mac has black title area as attached.
No debug start button and so on as attached.

Best regards.