Ajax callback for the style group dropdown.
public static function updateDefaultStyle(array $form, FormStateInterface $form_state) {
$group_select = $form_state
->getTriggeringElement();
// Gets the behavior plugin settings form.
$settings_form = NestedArray::getValue($form, array_slice($group_select['#array_parents'], 0, -2));
return $settings_form['groups_defaults'];
}