I was able to reproduce the error you mention. With a bit of debugging I realized the issue that was causing region not to be created was the
IsInDesignMode() method from the RegionManager class which needs the
Application.Current to be set.
The right way of launching the Shell is though the Application as follows:
That way the Bootstrapper is also ran and the
Application.Current will be set correctly. You will need to add references to the WindowsBase and PresentationFramework assemblies in your
WinForm launcher project.
Hope it helps!