fix(#362): improve error message for null HttpContext

src/JsonApiDotNetCore/Services/ScopedServiceProvider.cs

@@ -1,3 +1,4 @@
+using JsonApiDotNetCore.Internal;
 using Microsoft.AspNetCore.Http;
 using System;
 
@@ -23,6 +24,16 @@ public RequestScopedServiceProvider(IHttpContextAccessor httpContextAccessor)
         }
 
         /// <inheritdoc />
-        public object GetService(Type serviceType) => _httpContextAccessor.HttpContext.RequestServices.GetService(serviceType);
+        public object GetService(Type serviceType)
+        {
+            if (_httpContextAccessor.HttpContext == null)
+                throw new JsonApiException(500,
+                    "Cannot resolve scoped service outside the context of an HTTP Request.",
+                    detail: "If you are hitting this error in automated tests, you should instead inject your own "
+                            + "IScopedServiceProvider implementation. See the GitHub repository for how we do this internally. "
+                            + "https://github.com/json-api-dotnet/JsonApiDotNetCore/search?q=TestScopedServiceProvider&unscoped_q=TestScopedServiceProvider");
+
+            return _httpContextAccessor.HttpContext.RequestServices.GetService(serviceType);
+        }
     }
 }