Exception on startup and "add folders" when nuget dir does not exist

[mhusinsky]

Using Stride.ShaderExplorer might also make sense when Stride Studio is not installed (e.g. for browsing the VVVV Gamma shaders). However it might happen then that the Username/.nuget/packages dir does not exist -> then ShaderExplorer throws an exception on startup, or when an additional source folder is referenced:

unfortunately this also means that the additional folder will not be taken into account.

an easy workaround is to manually create the .nuget/packages folder, then everything works as expected.

however, a fix for the exception would be even nicer UX.