fix: remove document from dataset error when vector index npe (#489)

api/tasks/remove_document_from_index_task.py

@@ -41,6 +41,7 @@ def remove_document_from_index_task(document_id: str):
         kw_index = IndexBuilder.get_index(dataset, 'economy')
 
         # delete from vector index
+        if vector_index:
             vector_index.delete_by_document_id(document.id)
 
         # delete from keyword index