I am trying to implement an IVirtualPageController to serve some dynamic XML sitemaps. I have pretty much been following the documentation here, but I simply cannot get an Umbraco Context in the FindContent method.

Here is my controller, implementing IVirtualPageController:

public class SitemapController : UmbracoPageController, IVirtualPageController
{
    private readonly IUmbracoContextAccessor _umbracoContextAccessor;

    public SitemapController(ILogger<SitemapController> logger, ICompositeViewEngine compositeViewEngine, IUmbracoContextAccessor umbracoContextAccessor) : base(logger, compositeViewEngine)
    {
        _umbracoContextAccessor = umbracoContextAccessor;
    }

    public IPublishedContent FindContent(ActionExecutingContext actionExecutingContext)
    {
        IPublishedContent siteRoot = null;

        if (_umbracoContextAccessor.TryGetUmbracoContext(out var context))
        {
            siteRoot = context.Content.GetAtRoot().First();
        }

        return siteRoot;
    }

    [HttpGet]
    public ActionResult SitemapIndex()
    {
        //Removed for brevity

        Response.ContentType = "text/xml";
        return View("SitemapIndex", sitemapIndexViewModel);
    }
}

Here is the composer where I register the new route:

public class SitemapComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
    {
        builder.Services.Configure<UmbracoPipelineOptions>(options =>
        {
            options.AddFilter(new UmbracoPipelineFilter(nameof(SitemapController))
            {
                Endpoints = app => app.UseEndpoints(endpoints =>
                {
                    endpoints.MapControllerRoute("Sitemap Index", "/sitemapindex.xml", new { Controller = "Sitemap", Action = "SitemapIndex" });
                })
            });
        });
    }

}

The controller is being hit, and the FindContent method is being hit, but the Umbraco Context is always null.