Steps to reproduce:

1. Create a new project
2. Create a new script inside the project
3. Double-click the script to open it in VS for Mac
4. 2 errors are thrown and the C# project isn't generated (even though the script is opened correctly, the suggestions/autocomplete are not working for Unity classes):

NotSupportedException: Specified method is not supported.
SyntaxTree.VisualStudio.Unity.Bridge.ProjectSystem.UnitySolutionBuilder.ClassLibraryReference (System.String name) (at <70df2a6d49964eb58351ebd0338a7bed>:0)

Notes:
Trying to open C# project via Assets/Open C# project just throws the same error and doesn't open anything.
If you open a script on a project that has C# project files generated from an older Unity version, error is still thrown in console, but VS opens the project correctly and autocomplete works.

Reproduced with: 2018.3.0a4, 2018.3.0a6
Not reproduced with: 2018.3.0a3