[Blazor] [Fixes #11847] Renderer.DispatchEventAsync throws null reference exception if event handler throws synchronously (#12393)

[Blazor] [Fixes #11847] Renderer.DispatchEventAsync throws null
reference exception if event handler throws synchronously
* Returns after handling the exception.
* Adds a unit test and an E2E test to validate expected behavior.

src/Components/Components/src/Rendering/Renderer.cs

@@ -235,6 +235,7 @@ namespace Microsoft.AspNetCore.Components.Rendering
             catch (Exception e)
             {
                 HandleException(e);
+                return Task.CompletedTask;
             }
             finally
             {

src/Components/Components/test/RendererTest.cs

@@ -456,7 +456,7 @@ namespace Microsoft.AspNetCore.Components.Test
         }
 
         [Fact]
-        public async Task CanDispatchEventsToTopLevelComponents()
+        public void CanDispatchEventsToTopLevelComponents()
         {
             // Arrange: Render a component with an event handler
             var renderer = new TestRenderer();
@@ -482,12 +482,42 @@ namespace Microsoft.AspNetCore.Components.Test
             var renderTask = renderer.DispatchEventAsync(eventHandlerId, eventArgs);
             Assert.True(renderTask.IsCompletedSuccessfully);
             Assert.Same(eventArgs, receivedArgs);
-
-            await renderTask; // Does not throw
         }
 
         [Fact]
-        public async Task CanDispatchTypedEventsToTopLevelComponents()
+        public void DispatchEventHandlesSynchronousExceptionsFromEventHandlers()
+        {
+            // Arrange: Render a component with an event handler
+            var renderer = new TestRenderer {
+                ShouldHandleExceptions = true
+            };
+
+            var component = new EventComponent
+            {
+                OnTest = args => throw new Exception("Error")
+            };
+            var componentId = renderer.AssignRootComponentId(component);
+            component.TriggerRender();
+
+            var eventHandlerId = renderer.Batches.Single()
+                .ReferenceFrames
+                .First(frame => frame.AttributeValue != null)
+                .AttributeEventHandlerId;
+
+            // Assert: Event not yet fired
+            Assert.Empty(renderer.HandledExceptions);
+
+            // Act/Assert: Event can be fired
+            var eventArgs = new UIEventArgs();
+            var renderTask = renderer.DispatchEventAsync(eventHandlerId, eventArgs);
+            Assert.True(renderTask.IsCompletedSuccessfully);
+
+            var exception = Assert.Single(renderer.HandledExceptions);
+            Assert.Equal("Error", exception.Message);
+        }
+
+        [Fact]
+        public void CanDispatchTypedEventsToTopLevelComponents()
         {
             // Arrange: Render a component with an event handler
             var renderer = new TestRenderer();
@@ -513,12 +543,10 @@ namespace Microsoft.AspNetCore.Components.Test
             var renderTask = renderer.DispatchEventAsync(eventHandlerId, eventArgs);
             Assert.True(renderTask.IsCompletedSuccessfully);
             Assert.Same(eventArgs, receivedArgs);
-
-            await renderTask; // does not throw
         }
 
         [Fact]
-        public async Task CanDispatchActionEventsToTopLevelComponents()
+        public void CanDispatchActionEventsToTopLevelComponents()
         {
             // Arrange: Render a component with an event handler
             var renderer = new TestRenderer();
@@ -544,12 +572,10 @@ namespace Microsoft.AspNetCore.Components.Test
             var renderTask = renderer.DispatchEventAsync(eventHandlerId, eventArgs);
             Assert.True(renderTask.IsCompletedSuccessfully);
             Assert.NotNull(receivedArgs);
-
-            await renderTask; // does not throw
         }
 
         [Fact]
-        public async Task CanDispatchEventsToNestedComponents()
+        public void CanDispatchEventsToNestedComponents()
         {
             UIEventArgs receivedArgs = null;
 
@@ -586,8 +612,6 @@ namespace Microsoft.AspNetCore.Components.Test
             var renderTask = renderer.DispatchEventAsync(eventHandlerId, eventArgs);
             Assert.True(renderTask.IsCompletedSuccessfully);
             Assert.Same(eventArgs, receivedArgs);
-
-            await renderTask; // does not throw
         }
 
         [Fact]

src/Components/test/E2ETest/ServerExecutionTests/InteropReliabilityTests.cs

@@ -490,6 +490,29 @@ namespace Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests
             await ValidateClientKeepsWorking(Client, batches);
         }
 
+
+        [Fact]
+        public async Task EventHandlerThrowsSyncExceptionTerminatesTheCircuit()
+        {
+            // Arrange
+            var (interopCalls, dotNetCompletions, batches) = ConfigureClient();
+            await GoToTestComponent(batches);
+            var sink = _serverFixture.Host.Services.GetRequiredService<TestSink>();
+            var logEvents = new List<(LogLevel logLevel, string eventIdName, Exception exception)>();
+            sink.MessageLogged += (wc) => logEvents.Add((wc.LogLevel, wc.EventId.Name, wc.Exception));
+
+            // Act
+            await Client.ClickAsync("event-handler-throw-sync");
+
+            Assert.Contains(
+                logEvents,
+                e => LogLevel.Warning == e.logLevel &&
+                    "UnhandledExceptionInCircuit" == e.eventIdName &&
+                    "Handler threw an exception" == e.exception.Message);
+
+            await ValidateClientKeepsWorking(Client, batches);
+        }
+
         private Task ValidateClientKeepsWorking(BlazorClient Client, List<(int, int, byte[])> batches) =>
             ValidateClientKeepsWorking(Client, () => batches.Count);
 

src/Components/test/testassets/BasicTestApp/ServerReliability/ReliabilityComponent.razor

@@ -19,6 +19,8 @@
 <button id="triggerjsinterop-success" @onclick="@TriggerJSInteropSuccess">Trigger successfull JS interop callback</button>
 <button id="triggerjsinterop-failure" @onclick="@TriggerJSInteropFailure">Trigger error JS interop callback</button>
 
+<button id="event-handler-throw-sync" @onclick="@TriggerSyncException">Trigger sync exception</button>
+
 <button id="thecounter" @onclick="@IncrementCount">Click me</button>
 
 @code
@@ -33,6 +35,8 @@
         currentCount++;
     }
 
+    void TriggerSyncException() => throw new Exception("Handler threw an exception");
+
     async Task TriggerJSInterop()
     {
         try