Prevent NullReferenceException when disposing Context.Scope (#782).

src/Microsoft.AspNetCore.Hosting/Internal/HostingApplication.cs

@@ -80,7 +80,7 @@ public void DisposeContext(Context context, Exception exception)
                 }
             }
 
-            context.Scope.Dispose();
+            context.Scope?.Dispose();
 
             _httpContextFactory.Dispose(httpContext);
         }

test/Microsoft.AspNetCore.Hosting.Tests/HostingApplicationTests.cs

@@ -0,0 +1,51 @@
+// 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;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Hosting.Internal;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Features;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.ObjectPool;
+using Microsoft.Extensions.Options;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Hosting.Tests
+{
+    public class HostingApplicationTests
+    {
+        [Fact]
+        public void DisposeContextDoesNotThrowWhenContextScopeIsNull()
+        {
+            // Arrange
+            var httpContextFactory = new HttpContextFactory(new DefaultObjectPoolProvider(), Options.Create(new FormOptions()), new HttpContextAccessor());
+            var hostingApplication = new HostingApplication(ctx => Task.FromResult(0), new NullScopeLogger(), new NoopDiagnosticSource(), httpContextFactory);
+            var context = hostingApplication.CreateContext(new FeatureCollection());
+
+            // Act/Assert
+            hostingApplication.DisposeContext(context, null);
+        }
+
+        private class NullScopeLogger : ILogger
+        {
+            public IDisposable BeginScope<TState>(TState state) => null;
+
+            public bool IsEnabled(LogLevel logLevel) => true;
+
+            public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
+            {
+            }
+        }
+
+        private class NoopDiagnosticSource : DiagnosticSource
+        {
+            public override bool IsEnabled(string name) => true;
+
+            public override void Write(string name, object value)
+            {
+            }
+        }
+    }
+}