Fix: KB folder may not there while creating virtual file (#9431)

### What problem does this PR solve?

KB folder may not there while creating virtual file. #9423 

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/services/file_service.py

@@ -227,10 +227,13 @@ class FileService(CommonService):
         #     tenant_id: Tenant ID
         # Returns:
         #     Knowledge base folder dictionary
-        for root in cls.model.select().where((cls.model.tenant_id == tenant_id), (cls.model.parent_id == cls.model.id)):
-            for folder in cls.model.select().where((cls.model.tenant_id == tenant_id), (cls.model.parent_id == root.id), (cls.model.name == KNOWLEDGEBASE_FOLDER_NAME)):
-                return folder.to_dict()
-        assert False, "Can't find the KB folder. Database init error."
+        root_folder = cls.get_root_folder(tenant_id)
+        root_id = root_folder["id"]
+        kb_folder = cls.model.select().where((cls.model.tenant_id == tenant_id), (cls.model.parent_id == root_id), (cls.model.name == KNOWLEDGEBASE_FOLDER_NAME)).first()
+        if not kb_folder:
+            kb_folder = cls.new_a_file_from_kb(tenant_id, KNOWLEDGEBASE_FOLDER_NAME, root_id)
+            return kb_folder
+        return kb_folder.to_dict()
 
     @classmethod
     @DB.connection_context()
@@ -499,10 +502,9 @@ class FileService(CommonService):
     @staticmethod
     def get_blob(user_id, location):
         bname = f"{user_id}-downloads"
-        return  STORAGE_IMPL.get(bname, location)
+        return STORAGE_IMPL.get(bname, location)
 
     @staticmethod
     def put_blob(user_id, location, blob):
         bname = f"{user_id}-downloads"
-        return  STORAGE_IMPL.put(bname, location, blob)
-
+        return STORAGE_IMPL.put(bname, location, blob)