prevent exception when impersonation with token by guest is not successful

Author: i-just

src/controllers/UsersController.php

@@ -378,7 +378,7 @@ public function actionImpersonateWithToken(int $userId, int $prevUserId): ?Respo
         if (!$success) {
             $this->setFailFlash(Craft::t('app', 'There was a problem impersonating this user.'));
             Craft::error(sprintf('%s tried to impersonate userId: %s but something went wrong.',
-                $userSession->getIdentity()->username, $userId), __METHOD__);
+                $userSession->getIdentity()?->username ?? 'Unknown user', $userId), __METHOD__);
             return null;
         }