fix rerank mode is none (#7496)

api/core/app/app_config/easy_ui_based_app/dataset/manager.py

@@ -93,7 +93,7 @@ class DatasetConfigManager:
                     reranking_model=dataset_configs.get('reranking_model'),
                     weights=dataset_configs.get('weights'),
                     reranking_enabled=dataset_configs.get('reranking_enabled', True),
-                    rerank_mode=dataset_configs["reranking_mode"],
+                    rerank_mode=dataset_configs.get('rerank_mode', 'reranking_model'),
                 )
             )