Fix Postgres Not Null Violation

1. Use Postgres (in my case, I'm using 10.3)
2. Install Simple Sitemap
3. Generate or rebuild queue
4. You get a "Not Null Violation"

Drupal\Core\Database\IntegrityConstraintViolationException: SQLSTATE[23502]: Not null violation: 7 ERROR: null value in column "created" violates not-null constraint DETAIL: Failing row contains (43, simple_sitemap_elements, a:4:{s:4:"data";a:3:{s:4:"path";s:1:"/";s:8:"priority";s:3:"1.0"..., 0, null).: INSERT INTO queue (name, data, created) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2); Array ( ) in Drupal\Core\Database\Connection->handleQueryException() (line 683 of /app/drupal-8.6.3/core/lib/Drupal/Core/Database/Connection.php).