Argument #1 ($elements) must be of type array, null given, called in core layout_discovery.module

Problem/Motivation

Error met,

The website encountered an unexpected error. Try again later.
TypeError: Drupal\Core\Render\Element::children(): Argument #1 ($elements) must be of type array, null given, called 
in /web/core/modules/layout_discovery/layout_discovery.module on line 45 in Drupal\Core\Render\Element::children() 
(line 72 of core/lib/Drupal/Core/Render/Element.php).
template_preprocess_layout()
call_user_func_array() (Line: 261)
Drupal\Core\Theme\ThemeManager->render() (Line: 480)
Drupal\Core\Render\Renderer->doRender() (Line: 240)
Drupal\Core\Render\Renderer->render() (Line: 475)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 125)
__TwigTemplate_8f6677c23a6d65de338b6ae8876df912->doDisplay() (Line: 394)
Twig\Template->displayWithErrorHandling() (Line: 367)
Twig\Template->display() (Line: 379)
Twig\Template->render() (Line: 38)
Twig\TemplateWrapper->render() (Line: 39)
twig_render_template() (Line: 348)

The corresponding code in modules/layout_discovery/layout_discovery.module is

  // Create an attributes variable for each region.
  foreach (Element::children($variables['content']) as $name) {
    if (!isset($variables['content'][$name]['#attributes'])) {
      $variables['content'][$name]['#attributes'] = [];
    }
    $variables['region_attributes'][$name] = new Attribute($variables['content'][$name]['#attributes']);
  }

Steps to reproduce

Proposed resolution

Change to,

  if (isset($variables['content'])) {
    foreach (Element::children($variables['content']) as $name) {
      if (!isset($variables['content'][$name]['#attributes'])) {
        $variables['content'][$name]['#attributes'] = [];
      }
      $variables['region_attributes'][$name] = new Attribute($variables['content'][$name]['#attributes']);
    }
  }

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet