#null given in array_key_exists() (line 236 of modules/contrib/single_content_sync/src/ContentSyncHelper.php)

I added is_array in line 236
return is_array($allowed_entity_types) &&

can you provide more info, steps to reproduce?

seems like your config does not exist, if you upgraded from older version, did you run drush updb? did you export configs after that?