Parameter instance with value null

(and other design errors) when opening XSD files

The solution is found by doing this

on MS Development Network and StackOverflow. The workaround I found works that way:

Open the Project (or Website) Property Page.
Locate the section where you can choose the .NET Framework version (Start Options -> Build for most project types).
If your project is using a .NET Framework lower than 4.7.2, set it to 4.7.2 (in case you don’t have it installed, you might have to download it and restart Visual Studio afterwards).
If your project is using the .NET Framework 4.7.2, downgrade it to 4.6.1 (again, if you don’t have it download it and restart Visual Studio).

This should permanently fix your problem and let you open the XSD files in design mode.