fix: import null-success regression exception type

Author: KSemenenko

ManagedCode.Communication.Tests/AspNetCore/Extensions/ControllerExtensionsTests.cs

@@ -1,3 +1,4 @@
+using System;
 using ManagedCode.Communication.AspNetCore.Extensions;
 using ManagedCode.Communication.Constants;
 using ManagedCode.Communication.Tests.TestHelpers;