Null warning on end() Breadcrumb won't display correctly for role without proper permission

Problem/Motivation

This is cause by populating breadcrumb menu links which aren't accessible by the current user in whilst other links are. The inaccessible links returns NULL on end($menu_item['map']).

Steps to reproduce

1. Create contents and assign entityqueue.
2. Allow a new role to control access of entityqueue without giving the accessibility of other admin menu.
3. Masquerade as new role.
4. Enter the "Edit items" page of an entityqueue edit page. ( ex:
admin/structure/entityqueue/list/entityqueue_block_custom/subqueues/1/edit?destination=admin/structure/entityqueue).
5. See error message

Proposed resolution

Add condition to check if $menu_item['map'] and $last_item exists.