[Transform] NPE when updating a running transform with defer_validation set to true

[przemekwitek]

Elasticsearch Version

main

Installed Plugins

No response

Java Version

bundled

OS Version

MacOS

Problem Description

NPE is thrown when a running transform is updated with defer_validation flag set to true

Steps to Reproduce

1. Create a continuous transform
2. Start a transform
3. Update the transform with defer_validation set to true

Observe the NPE error.

Logs (if relevant)

org.elasticsearch.xpack.transform.integration.TransformDestIndexIT > testTransformDestIndexCreatedDuringUpdate_DeferValidation FAILED
    org.elasticsearch.client.ResponseException: method [POST], host [http://127.0.0.1:65232], URI [/_transform/test_dest_index_on_update/_update?defer_validation=true], status line [HTTP/1.1 500 Internal Server Error]
    {"error":{"root_cause":[{"type":"null_pointer_exception","reason":"Cannot invoke \"java.util.Map.entrySet()\" because \"mappings\" is null"}],"type":"null_pointer_exception","reason":"Cannot invoke \"java.util.Map.entrySet()\" because \"mappings\" is null"},"status":500}
        at __randomizedtesting.SeedInfo.seed([2D31F6CA040A6817:5DF2F169FC7E8B02]:0)
        at app//org.elasticsearch.client.RestClient.convertResponse(RestClient.java:351)
        at app//org.elasticsearch.client.RestClient.performRequest(RestClient.java:317)
        at app//org.elasticsearch.client.RestClient.performRequest(RestClient.java:292)
        at app//org.elasticsearch.xpack.transform.integration.TransformRestTestCase.updateTransform(TransformRestTestCase.java:414)
        at app//org.elasticsearch.xpack.transform.integration.TransformDestIndexIT.testTransformDestIndexCreatedDuringUpdate_DeferValidation(TransformDestIndexIT.java:173)

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)