MVC 5 Owin Facebook Auth results in Null Reference Exception

This probably is a bug in identity OWIN extension code. I can’t repro the issue as my facebook payload always returns a username field in json, which is missing from your fb response. I am not quite sure why it’s not there.

The code in identity owin extension method doesn’t have a null check for the identity’s name claim which is same as the username field. We have filed a bug for it internally.

In order to workaround this issue, could you try replacing your ExternalLoginCallback method with following code:

   [AllowAnonymous]
    public async Task<ActionResult> ExternalLoginCallback(string returnUrl)
    {
        var result = await AuthenticationManager.AuthenticateAsync(DefaultAuthenticationTypes.ExternalCookie);
        if (result == null || result.Identity == null)
        {
            return RedirectToAction("Login");
        }

        var idClaim = result.Identity.FindFirst(ClaimTypes.NameIdentifier);
        if (idClaim == null)
        {
            return RedirectToAction("Login");
        }

        var login = new UserLoginInfo(idClaim.Issuer, idClaim.Value);
        var name = result.Identity.Name == null ? "" : result.Identity.Name.Replace(" ", "");

        // Sign in the user with this external login provider if the user already has a login
        var user = await UserManager.FindAsync(login);
        if (user != null)
        {
            await SignInAsync(user, isPersistent: false);
            return RedirectToLocal(returnUrl);
        }
        else
        {
            // If the user does not have an account, then prompt the user to create an account
            ViewBag.ReturnUrl = returnUrl;
            ViewBag.LoginProvider = login.LoginProvider;
            return View("ExternalLoginConfirmation", new ExternalLoginConfirmationViewModel { UserName = name });
        }
    }

The code will set default user name as empty when there is no username back from facebook/google.