Getting Ajax when trying to Add Section to Library after 2.x upgrade

This seems to be broken after https://git.drupalcode.org/project/section_library/-/commit/f338ef84ae79... commit

Reducing to normal since this relates to an interaction with custom code.

I will point out that paragraph cloning was broken prior to 🐛 Error after importing section/template that contains blocks with paragraphs without available source content Active for paragraphs in inline blocks where the inline block was saved as part of a layout before creating a section template and then the original source block was deleted.

It looks like the hook is targeting a media entity. How does that interact with the paragraph and block entities? Is there a media reference field on the paragraph?

No Its not necessary that hook contain media entity.
If my hook is like below still its causing the failure

function custom_media_entity_bundle_field_info_alter(array &$fields, EntityTypeInterface $entity_type, string $bundle): ?array {
    $fields = [];
    return $fields;
}

And I found out why we are getting error here
The issue is in https://git.drupalcode.org/project/section_library/-/blob/2.0.0/src/Deep... line
$default_fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('paragraph', NULL);

So what I did I remove the above line and added below line within the foreach look just below it

$default_fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('paragraph', $paragraph->getType());

foreach ($value as $key => $item) {
        // createDuplicates() creates a duplicate with the same parent_id.
        // Our duplicate needs to be independent of original inline block id.
        $paragraph = $paragraph_storage->load($item['target_id']);
$default_fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('paragraph', $paragraph->getType());
....
....

Then Its working fine for me

So the problem is:
Since in hook_entity_bundle_field_info_alter() we are using typed parameter so whenever $default_fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('paragraph', NULL); line executes, the hook_entity_bundle_field_info_alter() hook also invoked and since 2nd parameter is passed NULL from section_library module. It breaks here due to type mismatched.

Thank you for that level of debugging and detail!

Does the attached MR address your issue?

Yes the changes in MR working for me.