Error during ezplatform:reindex with Elastic on Experience & Content edition.

Prereq.: Ibexa Content or Experience. Does not occur on Commerce via flex or meta.

Steps:

1. Start ES via e.g. docker run -itd --name ez-elastic -p 9200:9200 -p 9300:9300 -e "discovery.type=single-node" docker.elastic.co/elasticsearch/elasticsearch:7.7.1
2. Set in .env SEARCH_ENGINE=elasticsearch & ELASTICSEARCH_DSN=http://0.0.0.0:9200
3. Execute php bin/console ibexa:elasticsearch:put-index-template
4. Remove and clear cache.
5. Execute php bin/console ibexa:reindex

Actual result: Error occurs:

Purging index...
Re-creating index for 93 items across 2 iteration(s), using a single (current) process:
 1/2 [==============>-------------]  50% 2 secs/4 secs
In SesProductFieldMapper.php line 179:

  Notice: Trying to access array offset on value of type null