Notice: Undefined index: #summary_attributes in template_preprocess_details() (line 244 of core/includes/form.inc).

Drupal 8.6.x introduced the #summary_attributes property to details elements, and template_preprocess_details expects the property to exist in the render array (template_preprocess_details). BEF checkboxes are rendered in a details element and because #summary_attributes is not set the following warnings are given:

Notice: Undefined index: #summary_attributes in template_preprocess_details() (line 244 of core/includes/form.inc).
Warning: Invalid argument supplied for foreach() in Drupal\Core\Template\Attribute->__construct() (line 81 of core/lib/Drupal/Core/Template/Attribute.php).

It can be fixed by assigning an empty array to $form[$field_id]['#summary_attributes'] in exposedFormAlter()