fix: return message error in blocking mode (#657)

api/services/completion_service.py

@@ -390,7 +390,7 @@ class CompletionService:
                         result = json.loads(result)
                         if result.get('error'):
                             cls.handle_error(result)
-
+                        if 'data' in result:
                             return cls.get_message_response_data(result.get('data'))
             except ValueError as e:
                 if e.args[0] != "I/O operation on closed file.":  # ignore this error