fix: prevent insert_shared_chat_by_chat_id crash when chat is None (#20273)

Add null check after db.get(Chat, chat_id) before accessing chat.share_id. Returns None instead of crashing with AttributeError when chat doesn't exist.

backend/open_webui/models/chats.py

@@ -488,6 +488,9 @@ class ChatTable:
         with get_db_context(db) as db:
             # Get the existing chat to share
             chat = db.get(Chat, chat_id)
+            # Check if chat exists
+            if not chat:
+                return None
             # Check if the chat is already shared
             if chat.share_id:
                 return self.get_chat_by_id_and_user_id(chat.share_id, "shared", db=db)