Issue #3355182: Potentially Invalid array_search() call in Multiselect.php

Closes #3355182

src/Element/Multiselect.php

@@ -90,7 +90,7 @@ class Multiselect extends Select {
           case 'selected':
             if ($value_valid && ((!$value_is_array && (string) $element['#value'] === $key || ($value_is_array && in_array($key, $element['#value']))) || $empty_choice)) {
               if ($value_is_array) {
-                $options[array_search($key, $element['#value'])] = [
+                $options[$key] = [
                   'type' => 'option',
                   'value' => $key,
                   'label' => $choice,