Call to a member function getSetting() on null in Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver->getDerivativeDefinitions()

Problem/Motivation

I have been developing a site that uses migrate fairly heavily to migrate data from a D6 site to a D8 site. Migration has run just fine on the site until doing an upgrade to 8.5.4 - the migration runs in 8.5.3, but after upgrading to either 8.5.4 or 8.5.5 migration consistently is broken. The symptom, for example when running drush ms is

Error: Call to a member function getSetting() on null in             [error]
Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver->getDerivativeDefinitions()
(line 101 of
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate_drupal/src/Plugin/migrate/EntityReferenceTranslationDeriver.php)
#0
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php(101):
Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver->getDerivativeDefinitions(Array)
#1
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php(87):
Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator->getDerivatives(Array)
#2
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate/src/Plugin/MigrationPluginManager.php(256):
Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator->getDefinitions()
#3
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php(175):
Drupal\migrate\Plugin\MigrationPluginManager->findDefinitions()
#4
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate/src/Plugin/MigrationPluginManager.php(109):
Drupal\Core\Plugin\DefaultPluginManager->getDefinitions()
#5
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/modules/contrib/migrate_tools/migrate_tools.drush.inc(499):
Drupal\migrate\Plugin\MigrationPluginManager->createInstances(Array)
#6
/home/elections/domains/develections.cruiskeenconsulting.com/public_html/modules/contrib/migrate_tools/migrate_tools.drush.inc(149):
drush_migrate_tools_migration_list('')
#7 phar:///usr/bin/drush/includes/command.inc(422):
drush_migrate_tools_migrate_status()
#8 phar:///usr/bin/drush/includes/command.inc(231):
_drush_invoke_hooks(Array, Array)
#9 phar:///usr/bin/drush/includes/command.inc(199): drush_command()
#10 phar:///usr/bin/drush/lib/Drush/Boot/BaseBoot.php(67):
drush_dispatch(Array)
#11 phar:///usr/bin/drush/includes/preflight.inc(66):
Drush\Boot\BaseBoot->bootstrap_and_dispatch()
#12 phar:///usr/bin/drush/includes/startup.inc(462): drush_main()
#13 phar:///usr/bin/drush/includes/startup.inc(369):
drush_run_main(false, '/', 'Phar detected. ...')
#14 phar:///usr/bin/drush/drush(114): drush_startup(Array)
#15 /usr/bin/drush(10): require('phar:///usr/bin...')
#16 {main}.
Error: Call to a member function getSetting() on null in /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate_drupal/src/Plugin/migrate/EntityReferenceTranslationDeriver.php on line 101 #0 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php(101): Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver->getDerivativeDefinitions(Array)
#1 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php(87): Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator->getDerivatives(Array)
#2 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate/src/Plugin/MigrationPluginManager.php(256): Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator->getDefinitions()
#3 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php(175): Drupal\migrate\Plugin\MigrationPluginManager->findDefinitions()
#4 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate/src/Plugin/MigrationPluginManager.php(109): Drupal\Core\Plugin\DefaultPluginManager->getDefinitions()
#5 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/modules/contrib/migrate_tools/migrate_tools.drush.inc(499): Drupal\migrate\Plugin\MigrationPluginManager->createInstances(Array)
#6 /home/elections/domains/develections.cruiskeenconsulting.com/public_html/modules/contrib/migrate_tools/migrate_tools.drush.inc(149): drush_migrate_tools_migration_list('')
#7 phar:///usr/bin/drush/includes/command.inc(422): drush_migrate_tools_migrate_status()
#8 phar:///usr/bin/drush/includes/command.inc(231): _drush_invoke_hooks(Array, Array)
#9 phar:///usr/bin/drush/includes/command.inc(199): drush_command()
#10 phar:///usr/bin/drush/lib/Drush/Boot/BaseBoot.php(67): drush_dispatch(Array)
#11 phar:///usr/bin/drush/includes/preflight.inc(66): Drush\Boot\BaseBoot->bootstrap_and_dispatch()
#12 phar:///usr/bin/drush/includes/startup.inc(462): drush_main()
#13 phar:///usr/bin/drush/includes/startup.inc(369): drush_run_main(false, '/', 'Phar detected. ...')
#14 phar:///usr/bin/drush/drush(114): drush_startup(Array)
#15 /usr/bin/drush(10): require('phar:///usr/bin...')
#16 {main}
Error: Call to a member function getSetting() on null in Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver->getDerivativeDefinitions() (line 101 of /home/elections/domains/develections.cruiskeenconsulting.com/public_html/core/modules/migrate_drupal/src/Plugin/migrate/EntityReferenceTranslationDeriver.php).
Drush command terminated abnormally due to an unrecoverable error.   [error]

I'm not quite certain how to proceed at this point, this sounds quite a lot like it may be related to https://www.drupal.org/project/drupal/issues/2912348 → - but that appears to be fixed. If there is any help possible here I'd appreciate it.

Steps to reproduce

TBD

Proposed resolution

Add a check for NULL before using the returned array. The docs for \Drupal\Core\Entity\EntityFieldManagerInterface::getFieldDefinitions() doesn't state that NULL can be returned.

Remaining tasks

Manual testing - There are 3 reports of successful manual testing of the patch in #3, See #8, #10, #12, and #26.
Write a test - To do this a field needs to be created that does not have a definition. How to do that?
Review
Commit