NUnit 3.7.1 introduced changes that break NUnit tests executed with SpecFlow+ Runner when using NUnit’s Assert function. This results in an error similar to the following:
Object reference not set to an instance of an object.System.NullReferenceException: Object reference not set to an instance of an object. at NUnit.Framework.Assert.That[TActual](TActual actual, IResolveConstraint expression, String message, Object args)
This is a known issue. Until we are able to fix this, please stick to NUnit 3.6 if you are using SpecFlow+ Runner to execute your NUnit tests.