When logged in, the URL /account/login is still accessible (Fix https://github.com/btcpayserver/btcpayserver/issues/916)

BTCPayServer/Controllers/AccountController.cs

@@ -73,6 +73,8 @@ namespace BTCPayServer.Controllers
         [AllowAnonymous]
         public async Task<IActionResult> Login(string returnUrl = null)
         {
+            if (User.Identity.IsAuthenticated)
+                RedirectToLocal(returnUrl);
             // Clear the existing external cookie to ensure a clean login process
             await HttpContext.SignOutAsync(IdentityConstants.ExternalScheme);
 
@@ -647,7 +649,7 @@ namespace BTCPayServer.Controllers
 
         private IActionResult RedirectToLocal(string returnUrl)
         {
-            if (Url.IsLocalUrl(returnUrl))
+            if (!string.IsNullOrEmpty(returnUrl) && Url.IsLocalUrl(returnUrl))
             {
                 return Redirect(returnUrl);
             }