I’m not sure how much WP Debug Error notices are valued but I get a notice for every ACF call regarding line 160 in (pro version)/api/api-template.php (version 5.0.3):
if( isset($_GET[‘preview_id’]) )
{
$autosave = wp_get_post_autosave( $_GET[‘preview_id’] );
if( $autosave->post_parent == $post_id )
{
$post_id = intval( $autosave->ID );
}
}
Still the same in 5.1.4.
Now the error message points at line 2572 in api/api-helpers.php.
Conclusion: the function “wp_get_post_autosave” can return a boolean value. I think that’s the point.
Cheers