`ArgumentNullException` instead of branch names

[premun]

Does not work in Visual Studio 17.13 (Preview at the moment)

The repository is public, the piece of code associated is here: https://github.com/dotnet/arcade-services/blob/c64a725c034ee08fea78ae9c92826706e8f3eea5/src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrForwardFlower.cs#L315-L316

[neuecc]

thank you but in my environment(VS 17.13.0 Preview 3.0, language pack Japanese) can not reproduce it.