Avoid crashing on null references

[davidalpert]

I found this tool (and the related blog post https://www.jerriepelser.com/blog/analyze-dotnet-project-dependencies-part-2/) very helpful in tracking down some odd binding errors I was getting in a .NET Core 3.1 project which had been downgraded to .NET Core 2.2 and was still somehow referencing a transitive dependency on a .NET Core 3.1 library.

while using it I would offer two small improvements:

• promote the hard-coded solution file name to a required CLI arg;
• add some defensive null checking to avoid bailing partway through the report on a NullReferenceException