Fixes null ref in DatabaseErrorPageMiddleware.

Fix aspnet/Home#2820 - UseDatabaseErrorPage throws NullReferenceException when ef exception occur in background task
Fix aspnet/EntityFrameworkCore#9599 - Insert entity in non existing table throws NullException

Added null checks to async local access in event callback.

Related to aspnet/Home#2825

src/Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore/DatabaseErrorPageMiddleware.cs

@@ -78,6 +78,8 @@ namespace Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore
             _options = options.Value;
             _logger = loggerFactory.CreateLogger<DatabaseErrorPageMiddleware>();
 
+            // Note: this currently leaks if the server hosting this middleware is disposed.
+            // See aspnet/Home #2825
             DiagnosticListener.AllListeners.Subscribe(this);
         }
 
@@ -121,7 +123,6 @@ namespace Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore
                         else
                         {
                             var relationalDatabaseCreator = context.GetService<IDatabaseCreator>() as IRelationalDatabaseCreator;
-
                             if (relationalDatabaseCreator == null)
                             {
                                 _logger.NotRelationalDatabase();
@@ -216,15 +217,17 @@ namespace Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore
         {
             switch (keyValuePair.Value)
             {
+                // NB: _localDiagnostic.Value can be null when this middleware has been leaked.
+
                 case DbContextErrorEventData contextErrorEventData:
                     {
-                        _localDiagnostic.Value.Hold(contextErrorEventData.Exception, contextErrorEventData.Context.GetType());
+                        _localDiagnostic.Value?.Hold(contextErrorEventData.Exception, contextErrorEventData.Context.GetType());
 
                         break;
                     }
                 case DbContextTypeErrorEventData contextTypeErrorEventData:
                     {
-                        _localDiagnostic.Value.Hold(contextTypeErrorEventData.Exception, contextTypeErrorEventData.ContextType);
+                        _localDiagnostic.Value?.Hold(contextTypeErrorEventData.Exception, contextTypeErrorEventData.ContextType);
 
                         break;
                     }

test/Diagnostics.EFCore.FunctionalTests/DatabaseErrorPageMiddlewareTest.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Data.SqlClient;
+using System.Diagnostics;
 using System.Linq;
 using System.Net;
 using System.Net.Http;
@@ -16,7 +17,9 @@ using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.AspNetCore.Testing.xunit;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Internal;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 using Xunit;
@@ -140,6 +143,31 @@ namespace Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.Tests
             }
         }
 
+        [ConditionalFact]
+        [OSSkipCondition(OperatingSystems.Linux)]
+        [OSSkipCondition(OperatingSystems.MacOSX)]
+        public void No_exception_on_diagnostic_event_received_when_null_state()
+        {
+            using (var database = SqlServerTestStore.CreateScratch())
+            {
+                using (var server = SetupTestServer<BloggingContext, NoMigrationsMiddleware>(database))
+                {
+                    using (var db = server.Host.Services.GetService<BloggingContext>())
+                    {
+                        db.Blogs.Add(new Blog());
+
+                        try
+                        {
+                            db.SaveChanges();
+                        }
+                        catch (SqlException)
+                        {
+                        }
+                    }
+                }
+            }
+        }
+
         [ConditionalFact]
         [OSSkipCondition(OperatingSystems.Linux)]
         [OSSkipCondition(OperatingSystems.MacOSX)]