This error is actually at the end of a trail of errors from simply trying to install the 'Newtonsoft.Json' package for json serialisation/deserialisation (which seems a lot simpler than the native .NET libraries for the same).
The trail goes like this:
- attempt to install 'Newtonsoft.Json' package -> 'Newtonsoft.Json' already has a dependency defined for 'NETStandard.Library' error
- attempt to install 'NETStandard.Library' package -> 'NETStandard.Library' already has a dependency defined for 'Microsoft.NETCore.Platforms' error
- attempt to install 'Microsoft.NETCore.Platforms' package -> The 'Microsoft.NETCore.Platforms 2.0.1' package requires NuGet client version '2.12' or above, but the current NuGet version is '2.8.60723.765' error
- attempt to install 'NuGet.Cient' (v3 core library) -> installed no proble, but not change to previous error
- attempt to install 'NuGet.Build' (v2.12.0) -> aborts with message: Attempting to resolve dependency 'NuGet.CommandLine '
- attempt to install 'NuGet.CommandLine (v4.4.1) -> System.NullReferenceException: Object reference not set to an instance of an object error
I have installed the latest .Net Core SDK (v2.1.2)