Threading issue when using IViewComponentHelper to convert ViewComponent to HTML string

Question

I'm trying to use IViewComponentHelper in a multi threaded manner. But it's throwing :( (see full exception at bottom of post)

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.AspNetCore.Http.DefaultHttpContext.get_Items()

I have no idea why.

Here's my setup: (simplified for brevity)

Controller:

public async Task<IActionResult> Get()
{
    var pdfStream = await _pdfService.GenerateAsync();

    return File(pdfStream, "application/pdf");
}

PdfService:

public class PdfService
{
    private readonly PdfSDK _pdfSDK;

    public constructor(PdfSDK pdfSDK)
    {
        _pdfSDK = pdfSDK;
    }

    public async Task<Stream> GenerateAsync()
    {
        List<Task<PdfDocument>> tasks = await _context.Foos.Select(x => DoWorkAsync(x));

        var pdfs = await Task.WhenAll(tasks);
        var mergedPdf = _pdfSDK.MergePdfs(pdfs);

        return mergedPdf.Stream;
    }

    private async Task<PdfDocument> DoWorkAsync(Foo foo)
    {
        var html = await _renderViewComponentService.RenderViewComponentAsStringAsync<MyViewComponent>(foo);
        var document = await _pdfSDK.HtmlToDocumentAsync(html);

        return document;
    }
}

PdfSDK:

public class PdfSDK
{
    public async Task<PdfDocument> HtmlToDocumentAsync(string html)
    {
        using var pdfEngine = new PdfEngine();
        var pdf = await pdfEngine.HtmlAsPdfAsync(html);

        return pdf;
    }

    public PdfDocument MergePdfs(params PdfDocument[] pdfs)
    {
        var pdf = PdfDocument.Merge(pdfs);

        return pdf;
    }
}

MyViewComponent:

public class MyViewComponent : ViewComponent
{
    public IViewComponentResult Invoke(Foo args)
    {
        return View(args);
    }
}

Default.cshtml

@model Foo

<h1>Hello from @Foo.Id<h1>

RenderViewComponentService:

public class RenderViewComponentService
{
    private readonly IServiceProvider _serviceProvider;
    private readonly ITempDataProvider _tempDataProvider;
    private readonly IViewComponentHelper _viewComponentHelper;

    public RenderViewComponentService(
        IServiceProvider serviceProvider,
        ITempDataProvider tempDataProvider,
        IViewComponentHelper viewComponentHelper
    )
    {
        _serviceProvider = serviceProvider;
        _tempDataProvider = tempDataProvider;
        _viewComponentHelper = viewComponentHelper;
    }

    public async Task<string> RenderViewComponentAsStringAsync<TViewComponent>(object args)
        where TViewComponent : ViewComponent
    {
        var viewContext = GetFakeViewContext();
        (_viewComponentHelper as IViewContextAware).Contextualize(viewContext);

        // this appears to call InvokeAsync in TViewComponent, but it'll also call Invoke (synchronously) if it's implemented
        // see https://learn.microsoft.com/en-us/aspnet/core/mvc/views/view-components?view=aspnetcore-3.1#perform-synchronous-work
        var htmlContent = await _viewComponentHelper.InvokeAsync<TViewComponent>(args); // exception is thrown here!
        using var stringWriter = new StringWriter();
        htmlContent.WriteTo(stringWriter, HtmlEncoder.Default);
        var html = stringWriter.ToString();

        return html;
    }

    private ViewContext GetFakeViewContext(ActionContext actionContext = null, TextWriter writer = null)
    {
        actionContext ??= GetFakeActionContext();
        var viewData = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary());
        var tempData = new TempDataDictionary(actionContext.HttpContext, _tempDataProvider);

        var viewContext = new ViewContext(
            actionContext,
            NullView.Instance,
            viewData,
            tempData,
            writer ?? TextWriter.Null,
            new HtmlHelperOptions());

        return viewContext;
    }

    private ActionContext GetFakeActionContext()
    {
        var httpContext = new DefaultHttpContext
        {
            RequestServices = _serviceProvider,
        };

        var routeData = new RouteData();
        var actionDescriptor = new ActionDescriptor();

        return new ActionContext(httpContext, routeData, actionDescriptor);
    }

    private class NullView : IView
    {
        public static readonly NullView Instance = new NullView();
        public string Path => string.Empty;
        public Task RenderAsync(ViewContext context)
        {
            if (context == null) { throw new ArgumentNullException(nameof(context)); }
            return Task.CompletedTask;
        }
    }
}

Exception:

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.AspNetCore.Http.DefaultHttpContext.get_Items()
   at Microsoft.AspNetCore.Mvc.Routing.UrlHelperFactory.GetUrlHelper(ActionContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorPageActivator.<>c__DisplayClass4_0.<.ctor>b__0(ViewContext context)
   at Microsoft.Extensions.Internal.PropertyActivator`1.Activate(Object instance, TContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorPagePropertyActivator.Activate(Object page, ViewContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorPageActivator.Activate(IRazorPage page, ViewContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, Boolean invokeViewStarts)        
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context)
   at Microsoft.AspNetCore.Mvc.ViewComponents.ViewViewComponentResult.ExecuteAsync(ViewComponentContext context)
   at Microsoft.AspNetCore.Mvc.ViewComponents.DefaultViewComponentInvoker.InvokeAsync(ViewComponentContext context)
   at Microsoft.AspNetCore.Mvc.ViewComponents.DefaultViewComponentHelper.InvokeCoreAsync(ViewComponentDescriptor descriptor, Object arguments)
   at MyProject.Services.RenderViewComponentService.RenderViewComponentToStringAsync[TViewComponent](Object args) in MyProject\Services\RenderViewComponentService.cs:line ??
   at MyProject.Services.PdfService.DoWorkAsync(Foo foo) in MyProject\Services\PdfService.cs:line ??
   at MyProject.Services.PdfService.GenerateAsync() in MyProject\Services\PdfService.cs:line ??
   at MyProject.Controllers.MyController.Get() in MyProject\Controllers\MyController.cs:line ??
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfIActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker 
invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task 
lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)    
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

I think the problem is how I Contextualize the IViewComponentHelper. But I'm at a loss on how else to do it.

Can you spot anything I'm doing wrong?