Issue #3160580 by Kirst25: Secondary div hidden when all secondary are collapsible

src/Plugin/better_exposed_filters/filter/FilterWidgetBase.php

@@ -213,7 +213,7 @@ abstract class FilterWidgetBase extends BetterExposedFiltersWidgetBase implement
 
       if ($is_secondary) {
         // Move secondary elements.
-        $form[$field_id . '_collapsible']['#group'] = 'secondary';
+        $this->addElementToGroup($form, $form_state, $field_id . '_collapsible', 'secondary');
       }
     }