Fix request delegate analyzer error on null return type (#44534)

Author: JamesNK

src/Framework/AspNetCoreAnalyzers/src/Analyzers/Http/RequestDelegateReturnTypeAnalyzer.cs

@@ -59,7 +59,10 @@ public override void Initialize(AnalysisContext context)
                             var resolvedOperation = WalkDownConversion(returnedValue);
                             var returnType = resolvedOperation.Type;
 
-                            if (SymbolEqualityComparer.Default.Equals(returnType.OriginalDefinition, wellKnownTypes.TaskOfT))
+                            // Return type could be null if:
+                            // 1. The method returns null.
+                            // 2. The method throws an exception.
+                            if (returnType != null && SymbolEqualityComparer.Default.Equals(returnType.OriginalDefinition, wellKnownTypes.TaskOfT))
                             {
                                 AddDiagnosticWarning(context, anonymousFunction.Syntax.GetLocation(), returnType);
                                 return;

src/Framework/AspNetCoreAnalyzers/test/Http/RequestDelegateReturnTypeAnalyzerTests.cs

@@ -10,6 +10,42 @@ namespace Microsoft.AspNetCore.Analyzers.Http;
 
 public class RequestDelegateReturnTypeAnalyzerTests
 {
+    [Fact]
+    public async Task AnonymousDelegate_RequestDelegate_ThrowError_NoDiagnostics()
+    {
+        // Arrange & Act & Assert
+        await VerifyCS.VerifyAnalyzerAsync(@"
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.Use(async (HttpContext context, Func<Task> next) =>
+{
+    context.SetEndpoint(new Endpoint(c => throw new Exception(), EndpointMetadataCollection.Empty, ""Test""));
+    await next();
+});
+");
+    }
+
+    [Fact]
+    public async Task AnonymousDelegate_RequestDelegate_ReturnNull_NoDiagnostics()
+    {
+        // Arrange & Act & Assert
+        await VerifyCS.VerifyAnalyzerAsync(@"
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.Use(async (HttpContext context, Func<Task> next) =>
+{
+    context.SetEndpoint(new Endpoint(c => null, EndpointMetadataCollection.Empty, ""Test""));
+    await next();
+});
+");
+    }
+
     [Fact]
     public async Task AnonymousDelegate_RequestDelegate_ReturnType_EndpointCtor_ReportDiagnostics()
     {