C# Asp.Net Core Middleware - Page Loading Time

I have implemented a middleware class that determines the elapsed time that a page took to load.

public sealed class PageLoadTimeMiddleware
{
    private readonly RequestDelegate _next;

    public PageLoadTimeMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        var stopwatch = new Stopwatch();
        stopwatch.Start();

        context.Items["Stopwatch"] = stopwatch;

        await _next(context);

        stopwatch.Stop();
        var elapsedMilliseconds = stopwatch.ElapsedMilliseconds;

        context.Response.Headers.Add("X-PageLoadTime", elapsedMilliseconds.ToString());
    }
}

In my Program.cs file, I configure the middleware to run:

app.UseMiddleware<PageLoadTimeMiddleware>();

However, when I run the web application, it gives me the following error page:

If I disable the middleware, the web application runs perfectly.

How can I debug this?

Can you provide any insight into why this might be happening?

FWIW, I am calling lots of different things in my Program.cs file (both before and after this middleware is configured).

Thanks

UPDATE: I've extracted this code out into a fresh "out of the box" C# ASP.Net MVC Core web application, and the result is still the same. The application just hangs when I try to run it.