[BUG] Query with UNION operator fails with NullPointerException

What is the bug?
Present in OpenSearch 2.12.0, 2.11.0, 1.3.10
The documentation states that Union queries are supported. However, when attempting a simple union query, the request fails with:

{ "error": { "reason": "There was internal problem at backend", "details": "", "type": "NullPointerException" }, "status": 500 }

The query returns the expected result when ?format=csv is added to the request URL.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Index data into OpenSearch (for this example, I am using simple novels/song data)
2. perform query involving union:
   { "query": "SELECT title FROM novels UNION SELECT title FROM songs" }
3. NullPointerException error returned

What is the expected behavior?
Union operator to work as expected. In above example adding the request call to be ''http://localhost:9200/_plugins/_sql?format=csv" returns:

title
Animal Farm
The Great Gatsby
Catcher In The Rye
Bohemian Rhapsody

What is your host/environment?
Present in OpenSearch 2.12, 2.11 and 1.3.10

Do you have any screenshots?
Error:

As expected (When adding ?format=csv to request URL):

Do you have any additional context?
Add any other context about the problem.

Start at the OpenSearch SQL plugin's _plugins/_sql endpoint and compare the default response path with the format=csv path for the UNION query shown. Reproduce the NullPointerException with the JSON response, then verify that the same UNION query returns its expected rows without requiring the CSV parameter.