Hello,
CreateParallelContext() can return a context with seg == NULL. That
causes CREATE INDEX to segfault. Instead, it should fall back to
non-parallel build. See attached.
This probably explains a segfault reported over on pgsql-general[1].
[1] https://www.postgresql.org/message-id/CA%2BhUKG%2BcfjHy63hXEOc-CRZEPcUhu9%3DP3gKk_W9OiXzj-dfV_g%40mail.gmail.com