🇺🇸United States @texasintegratedservices

In core/themes/claro/claro.theme
Change Line 1006

// Remove 'container-inline' class from the main attributes and add a flag
// instead.
// @todo remove this after https://www.drupal.org/node/3059593 → has been
// resolved.
if (!empty($variables['attributes']['class'])) {
if(is_array($variables['attributes']['class'])){ <-------------------Added
$container_inline_key = array_search('container-inline', $variables['attributes']['class']);

if ($container_inline_key !== FALSE) {
unset($variables['attributes']['class'][$container_inline_key]);
$variables['inline_items'] = TRUE;
}
}<-------------------Added
}