Please give us a description of what happened.

Hey team,

In this file https://plugins.trac.wordpress.org/browser/wordpress-seo/trunk/inc/sitemaps/class-post-type-sitemap-provider.php#L615 when you call get_permalink($post);

$post is not a WP_Post or and ID but your Stdclass.
In the function get_permalink() the final return has a filter :

/**
* @param string $permalink The post’s permalink.
* @param WP_Post $post The post in question.
* @param bool $leavename Whether to keep the post name.
*/
apply_filters( 'post_link', $permalink, $post, $leavename );

If any plugins or custom code hook ‘post_link’ after WordPress SEO it can generate a fatal error.
Could you modify your code to put an ID or an WP_Post instance ?
Best,

Please describe what you expected to happen and why.

In https://plugins.trac.wordpress.org/browser/wordpress-seo/trunk/inc/sitemaps/class-post-type-sitemap-provider.php#L615, on your hook : apply_filters( 'wpseo_xml_sitemap_post_url', get_permalink( $post ), $post );
In get_permalink() $post must be an ID or au WP_Post. In your code you put an StdClass

How can we reproduce this behavior?

1. Activate Yoast SEO
2. Add the code:

add_filter( 'post_link', function( string $permalink, WP_Post $post, bool $leavename ){ 
    /* my custom code */
    return $permalink;
} 10, 3  );

3. Visit post-sitemap.xml. Fatal Error is triggered because $post is an StdClass

Technical info

• If relevant, which editor is affected (or editors):

• Classic Editor
• Gutenberg
• Classic Editor plugin

• Which browser is affected (or browsers):

• Chrome
• Firefox
• Safari
• Other

Used versions

• WordPress version: 5.5.3
• Yoast SEO version: 15.2.1

• Tested with theme: Twenty Seventeen