PHP 8 TypeError: method_exists()

Problem/Motivation

method_exists() did not throw exceptions on unexpected values.

TypeError: method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given in method_exists() (line 123 of /web/modules/contrib/simple_sitemap/src/Plugin/simple_sitemap/UrlGenerator/CustomUrlGenerator.php)

Steps to reproduce

add custom link :/ admin/config/search/simplesitemap/custom
Generate sitemap

Proposed resolution

replace in CustomUrlGenerator.php line 123

'lastmod' =>method_exists($entity, 'getChangedTime') ?
        date('c', $entity->getChangedTime()) : NULL,

by

'lastmod' =>($entity !== NULL && method_exists($entity, 'getChangedTime')) ?
        date('c', $entity->getChangedTime()) : NULL,