Fix exceptions for non-existing entity in migrate destination plugin rollback()

Coincidentally, just struggling with this same problem this afternoon, migrating using migrate_source_csv. (Most of the server OSs that run D6 sites are hitting EOL, must be that time of the year...)

Running a "drush mr" on the content type means that the attached paragraphs cannot be rolled back (with this identical error: Line 144 of EntityReferenceRevisions.php).

You can run

drush mrs {paragraph_migration_id}
drush mi {paragraph_migration_id} --update 

to work around the error (re-importing the entire paragraphs).

EDIT: Let me try this patch out...

The patch works lovely as best I can tell. No clue about that Test error - my PHP Unit is throwing an error of:

$ ~/.config/composer/vendor/bin/phpunit ../modules/entity_reference_revisions/src/Tests/EntityReferenceRevisionsDiffTest.php 
Class '../modules/entity_reference_revisions/src/Tests/EntityReferenceRevisionsDiffTest' could not be found in '/var/www/drupal8/test2.qualitycraft.com/public_html/modules/entity_reference_revisions/src/Tests/EntityReferenceRevisionsDiffTest.php'.

I'm also running into this problem when migrating data into D8 from an external not Drupal database. I can also confirm, that the patch solves the error when doing a rollback of the 'entity_reference_revisions:paragraph' migration (that is the first step of the migration as described by svendecabooter).

But it seems to me that the problem goes deeper: With the rollback of the 'entity:node' (that is the second step as described by svendecabooter) the paragraphs are deleted but migration is not informed about this (no update of the migrate_map). So migration still believes that the paragraph data is present (you can see in the migrate status for the first step that the data is marked as imported). I need to do a rollback of the first step, before I restart the second step migration, although migration status tells me that everything is imported. Otherwise the second step migration will run (and migration_dependencies is also not working, because migrate_map tells that everything is imported) but it will not apply the paragraphs data (as this data is not present anymore). As TrevorBradley pointed out, you need a re-import of the entire paragraphs. This may be no problem as long as you group your migrations and run rollback and migration for the whole group, but handling a lot of data with many migrations (and being in the process of testing the migration steps) I usually start and rollback migrations individually. In such a case the data can get a little bit of a mess if you not pay attention of this rollback relation between the first and the second step. Maybe someone has an answer for this problem (somewhere migrate_map needs to be updated)?

Committed, thx. :-)