attachment_fields_to_save first parameter is null

I have the following code in a self-written plugin. In the function saveHomepageGalleryCheckbox the first param ($post) is always null. Did I something wrong? The second parameter ($attachment) has the correct value.

• Wordpress Version: 5.3.2
• PHP Version: 7.3.15

Code:

class MediaView
{
  private $key = 'include_in_homepage_gallery';

  public function __construct()
  {
    add_filter('attachment_fields_to_edit', [$this, 'renderHomepageGalleryCheckbox'], 10, 2);
    add_filter('attachment_fields_to_save', [$this, 'saveHomepageGalleryCheckbox'], 10, 2);
  }

  public function renderHomepageGalleryCheckbox(array $fields, WP_Post $post)
  {
    $meta = get_post_meta($post-ID, $this-key, true);
    $checked = (boolval($meta) === true) ? 'checked="checked"' : '';
    $id = "attachments[{$post-ID}][{$this-key}]";

    $fields[$this-key] = [
      'label' = 'Homepage Gallery',
      'input' = 'html',
      'html' = "input type=\"checkbox\" name=\"$id\" id=\"$id\" value=\"1\" {$checked} /"
    ];

    return $fields;
  }

  public function saveHomepageGalleryCheckbox($post, array $attachment) // Here is $post === null
  {
    if (isset($attachment[$this-key])) update_post_meta($post['ID'], $this-key, 1);
    else update_post_meta($post['ID'], $this-key, 0);

    return $post;
  }
}