Issue #3387157 by bkildow, ltrain, Grimreaper: Multivalued fields throw an error

Closes #3387157

gin_lb.module

@@ -13,6 +13,7 @@ use Drupal\views\ViewExecutable;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Url;
 use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Template\Attribute;
 
 /**
  * Implements hook_preprocess_form_element().
@@ -955,7 +956,7 @@ function gin_lb_preprocess_field_multiple_value_form(&$variables) {
         '#type' => 'html_tag',
         '#tag' => 'h4',
         '#value' => $variables['element']['#title'],
-        '#attributes' => $header_attributes,
+        '#attributes' => new Attribute($header_attributes),
       ];
 
       if ($variables['disabled']) {