[Xwt] If the native window is null, return null

This is a more logical behaviour and prevents us from hitting silly
null reference exceptions later on in the apps lifespan.

This probably explains some mysterious null reference exceptions we've
been seeing for the last few weeks.
[Mac] Don't depend on view.Layer for BackgroundColor since we're not using CoreAnimation

NSView.Layer is only set if using CoreAnimation and WantsLayer is set to True.
Instead, Canvas will manually fill its background color before calling OnDraw.
Other widgets that need to respect BackgroundColor will likely need to do something similar.
Merge branch 'master' into xwt-next

Conflicts:
	Xwt/Xwt.Engine/WidgetRegistry.cs