MAPG-199 fix issues found by PHPStan

src/Controller/UserController.php

@@ -210,7 +210,7 @@ class UserController implements ISecured
         return new JsonContent(['success' => true]);
     }
 
-    private function confirmUserIdentity(User $user, ?DateTime $authenticatedWithGoogleUntil, ?string $password, &$error): bool
+    private function confirmUserIdentity(User $user, ?DateTime $authenticatedWithGoogleUntil, ?string $password, string &$error): bool
     {
         if ($authenticatedWithGoogleUntil !== null && $authenticatedWithGoogleUntil > new DateTime()) {
             return true;