Issue #3181173 by longwave, pefferen, JD_1, larowlan: Drupal 8.9.7 breaks Book...

   * {@inheritdoc}

   */

  public function validateForm(array &$form, FormStateInterface $form_state) {

    $child_type = array_filter($form_state->getValue('book_child_type'));

    $child_type = $form_state->getValue('book_child_type');

    if ($form_state->isValueEmpty(['book_allowed_types', $child_type])) {

      $form_state->setErrorByName('book_child_type', $this->t('The content type for the %add-child link must be one of those selected as an allowed book outline type.', ['%add-child' => $this->t('Add child page')]));

    }

    $this->config('book.settings')

    // Remove unchecked types.

      ->set('allowed_types', $allowed_types)

      ->set('child_type', array_filter($form_state->getValue('book_child_type')))

      ->set('child_type', $form_state->getValue('book_child_type'))

      ->save();

    parent::submitForm($form, $form_state);

    $this->assertText($this->book->label(), 'Unpublished book with "Show block only on book pages" book navigation settings.');

  }

  /**

   * Tests that the book settings form can be saved without error.

   */

  public function testSettingsForm() {

    $this->drupalLogin($this->adminUser);

    $this->drupalPostForm('admin/structure/book/settings', [], 'Save configuration');

  }

}