问当访问一个以数组作为值的键时，为什么var_dump显示NULL？

EN

object(WP_Theme)#795 (12) {
  ["update"]=>
  bool(false)
  ["theme_root":"WP_Theme":private]=>
  string(56) "/Users/tygoss/Projects/QuickPress/prod/wp-content/themes"
  ["headers":"WP_Theme":private]=>
  array(11) {
    ["Name"]=>
    string(10) "Black Jane"
    ["ThemeURI"]=>
    string(0) ""
    ["Description"]=>
    string(187) "Black Jane is a clean SuevaFree child theme with an optional slideshow on homepage, a new header layout in addition to the five header layouts available on SuevaFree and new Google Fonts."
    ["Author"]=>
    string(16) "ThemeinProgress."
    ["AuthorURI"]=>
    string(31) "https://www.themeinprogress.com"
    ["Version"]=>
    string(5) "1.0.4"
    ["Template"]=>
    string(9) "suevafree"
    ["Status"]=>
    string(0) ""
    ["Tags"]=>
    string(302) "blog, e-commerce, photography, custom-background, custom-colors, custom-header, custom-logo, custom-menu, featured-images, flexible-header, footer-widgets, post-formats, right-sidebar, sticky-post, theme-options, threaded-comments, translation-ready, one-column, two-columns, three-columns, grid-layout"
    ["TextDomain"]=>
    string(10) "black-jane"
    ["DomainPath"]=>
    string(10) "/languages"
  }
  ["name_translated":"WP_Theme":private]=>
  NULL
  ["errors":"WP_Theme":private]=>
  NULL
  ["stylesheet":"WP_Theme":private]=>
  string(10) "black-jane"
  ["template":"WP_Theme":private]=>
  string(9) "suevafree"
}

    $all_themes = wp_get_themes();
    foreach ($all_themes as $key => $value) {
      echo '<pre>';
      var_dump($value->headers);
      echo '</pre>';
    }