Net Core ILogger Value cannot be null

You have to initialize the logger variable with either an implementation of ILogger like ConsoleLogger or something similar. At the moment you aren't assigning any value to your test logger variable.

I would recommend passing in a mock of the logger because you're unit testing and probably don't want to test the logger itself. So use FakeItEasy or some other Mocking-Libary and create a faked instance over A.Fake<ILogger>()