I'm running multi-site with several related sites involved. In creating a master menu into all the sites using wp_list_pages I ran into a problem with one because the
$current_page number didn't exist in the specific site.
wp-includes/post-template.php was throwing a warning in the start_el function of class Walker at line 1059 because
$_current_page results in NULL.
I worked around the issue by changing line 1059 as follows:
if (in_array( $page->ID, $_current_page->ancestors ) )
if (is_object($_current_page) && in_array( $page->ID, $_current_page->ancestors ) )
It may be an unusual situation but I have posted it just in case someone else encounters it. It might also be beneficial for future use.