Drupal 8.9.7 breaks Book Settings Form Validation

I need to revert the changes which were made here:

https://git.drupalcode.org/project/drupal/commit/cb2e043f53fcf05dd20d164...

Error which occurs:

"The content type for the Add child page link must be one of those selected as an allowed book outline type."

I did some more research...

$child_types is empty after the following line:

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $child_type = array_filter($form_state->getValue('book_child_type'));

array_filter() is not working, because "$form_state->getValue('book_child_type')" is just a string of the content type like "book"

And I think it was mentioned here:

https://www.drupal.org/project/drupal/issues/315176#comment-12171773

The code is http://cgit.drupalcode.org/drupal/tree/core/modules/book/src/Form/BookSe...

allowed_types - checkboxes are filtered already
child_type is radios so no filtering required

So #27 is right patch that removes useless remains

So it only should be this patch be applied:
https://www.drupal.org/files/issues/315176-array_filter_remains-27.patch

Hi larowlan,

At the moment (with Drupal 8.9.7 and later) no one can activate a content type to create books or change the book settings.

The reason is the mentioned error in the form of the bool settings. Please see the attached screenshot.

It seems to me that the array_filter() function is killing the provided string, because it is just a string, not an array.

In the linked issue the comment (https://www.drupal.org/project/drupal/issues/315176#comment-12171773) reverts the patches which was committed to the core.

BR
Daniel

@pefferen

Perfect! Exactly what I did to fix this problem.

Is working for me, too.