add type check for upstream

backend/config.py

@@ -80,7 +80,7 @@ class ProxyElem:
         clean_urls = []
         link_requests: list[ProxyCreateLink] = []
         for url in urls:
-            clean_urls.append(url.upstream)
+            clean_urls.append(url.upstream if isinstance(url.upstream, str) else None)
         if not isinstance(proxy_server, str):
             return clean_urls