❔ Callback Url Always Null

        public IActionResult ExternalLogin(string provider, string returnurl = null)
        {
            var redirect = Url.Action("ExternalLoginCallback", "Account", new { ReturnUrl = returnurl });
            var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirect);
            return Challenge(properties, provider);
        }

        public async Task<IActionResult> ExternalLoginCallback(string returnurl = null, string remoteError = null)
        {
            if(remoteError != null)
            {
                ModelState.AddModelError(string.Empty, "Error from external provider");
                return View("Login");
            }
            var info = await _signInManager.GetExternalLoginInfoAsync();
            if(info == null)
            {
                return RedirectToAction("Login");
            }
            var result = await _signInManager.ExternalLoginSignInAsync(info.LoginProvider, info.ProviderKey, isPersistent: false);
            if (result.Succeeded)
            {
                await _signInManager.UpdateExternalAuthenticationTokensAsync(info);
                return LocalRedirect(returnurl);
            }
            else
            {
                ViewData["ReturnUrl"] = returnurl;
                ViewData["ProviderDisplayName"] = info.ProviderDisplayName;
                var email = info.Principal.FindFirstValue(ClaimTypes.Email);
                return View("ExternalLoginConfirmation", new ExternalLoginViewModel { Email = email });
            }
        }

        public IActionResult ExternalLogin(string provider, string returnurl = null)
        {
            var redirect = Url.Action("ExternalLoginCallback", "Account", new { ReturnUrl = returnurl });
            var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirect);
            return Challenge(properties, provider);
        }

        public async Task<IActionResult> ExternalLoginCallback(string returnurl = null, string remoteError = null)
        {
            if(remoteError != null)
            {
                ModelState.AddModelError(string.Empty, "Error from external provider");
                return View("Login");
            }
            var info = await _signInManager.GetExternalLoginInfoAsync();
            if(info == null)
            {
                return RedirectToAction("Login");
            }
            var result = await _signInManager.ExternalLoginSignInAsync(info.LoginProvider, info.ProviderKey, isPersistent: false);
            if (result.Succeeded)
            {
                await _signInManager.UpdateExternalAuthenticationTokensAsync(info);
                return LocalRedirect(returnurl);
            }
            else
            {
                ViewData["ReturnUrl"] = returnurl;
                ViewData["ProviderDisplayName"] = info.ProviderDisplayName;
                var email = info.Principal.FindFirstValue(ClaimTypes.Email);
                return View("ExternalLoginConfirmation", new ExternalLoginViewModel { Email = email });
            }
        }