fix: #3565121 Warning: Undefined property: Drupal\Core\Field\EntityReferenceFieldItemList:: in domain_source_get

In the spirit of Paragraphs MR 221 commit 23751df4 we avoid magic methods altogether. I don't understand why this works, but it does. The changelog for Paragraphs release 8.x-1.20 says this change will "Avoid fiber related test failure in Drupal 11.3" and the analogous change here does the trick.

Closes #3565121