If you have a content type that references other items of the same type, the referenced items may not be available when the parent is migrated. Drupal migrations can create stubbed entities for these items using the Migration Lookup process plugin. If your migration has a migration lookup that references itself, it can create a stub and migrate the remaining content when it reaches that item in the migration. If you do this, add a default title to your title field, so it can create the entity without error.