Issue #3178778 by Neslee Canil Pinto: Undefined index #multiple

includes/better_exposed_filters.theme.inc

@@ -215,7 +215,7 @@ function template_preprocess_bef_links(array &$variables) {
     // with the entire options array and remove items from it as needed.
     $urlOptions = $url->getOptions();
 
-    if ($element['#multiple']) {
+    if (isset($element['#multiple'])) {
       $newQuery = isset($existing_query[$name]) ? $existing_query[$name] : [];
       if (in_array($optionValue, $selectedValues)) {
         // Allow users to toggle an option using the same link.