Issue #3136406 by Spokje, codersukanta, IJsbrandy, atul4drupal, MahtabAlam, BalajiDS: Argument 1 passed to Drupal\Core\Form\SubformState::createForSubform() must be of the type array, null given

Issue #2851504 by alexpott, vomiand, Vj, BalajiDS, cilefen, tamasd: "Illegal choice 0 in Book element" when switching the book outline field from anything to "- None - "