Add-Migration Value cannot be null. Parameter name: language

This is issue #11075. The version of your tool doesn't match the version of your runtime. Ensure you've updated every Microsoft.EntityFrameworkCore package to 2.1.0-preview1-final.

It's related to the EF Core not EF6, it looks like there is a bug in the 2.1.0.preview1-final version.

• Check the version you are running with dotnet ef --version
• Downgrade to the Microsoft.EntityFrameworkCore.Design 2.0.1 version.