Thank you for your reply.
I search in git and found this issue null value in SearchResponse.hits.hits.sort causes parsing failure · Issue #66 · elastic/elasticsearch-java · GitHub
It seems to be resolved in 7.16.2 according to this thread Failure with sorts and new co.elastic.clients:elasticsearch-java client - #11 by swallez
But I am using 7.17.4...it is still happening.
Hope @swallez can comment on this?