Minor fix to DefaultLinkGenerator and enabled constraint related tests in DefaultLinkGeneratorTest

src/Microsoft.AspNetCore.Routing/DefaultLinkGenerator.cs

@@ -115,7 +115,7 @@ namespace Microsoft.AspNetCore.Routing
             {
                 var matchProcessorReference = endpoint.MatchProcessorReferences[i];
                 var parameter = endpoint.ParsedTemplate.GetParameter(matchProcessorReference.ParameterName);
-                if (parameter.IsOptional && !routeValues.ContainsKey(parameter.Name))
+                if (parameter != null && parameter.IsOptional && !routeValues.ContainsKey(parameter.Name))
                 {
                     continue;
                 }

test/Microsoft.AspNetCore.Routing.Tests/RouteTest.cs

@@ -9,6 +9,7 @@ using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Routing.Constraints;
 using Microsoft.AspNetCore.Routing.Internal;
+using Microsoft.AspNetCore.Routing.TestObjects;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
@@ -1843,21 +1844,5 @@ namespace Microsoft.AspNetCore.Routing
 
             return new DefaultInlineConstraintResolver(routeOptions.Object);
         }
-
-        private class CapturingConstraint : IRouteConstraint
-        {
-            public IDictionary<string, object> Values { get; private set; }
-
-            public bool Match(
-                HttpContext httpContext,
-                IRouter route,
-                string routeKey,
-                RouteValueDictionary values,
-                RouteDirection routeDirection)
-            {
-                Values = new RouteValueDictionary(values);
-                return true;
-            }
-        }
     }
 }

test/Microsoft.AspNetCore.Routing.Tests/TestObjects/CapturingConstraint.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.Routing.TestObjects
+{
+    internal class CapturingConstraint : IRouteConstraint
+    {
+        public IDictionary<string, object> Values { get; private set; }
+
+        public bool Match(
+            HttpContext httpContext,
+            IRouter route,
+            string routeKey,
+            RouteValueDictionary values,
+            RouteDirection routeDirection)
+        {
+            Values = new RouteValueDictionary(values);
+            return true;
+        }
+    }
+}