PHP Warning: ltrim() expects parameter 1 to be string, object given

Hi,

Another PHP warning in version 1.0.4 which pop-ups about every 4 hours.
Can you fix this in the next version release? Thanks.

Warning: ltrim() expects parameter 1 to be string, object given in /wp-includes/formatting.php on line 4309

Stack Trace

1.  ltrim(stdClass)
    /wp-includes/formatting.php:4309
2.  esc_url(stdClass)
    /wp-content/plugins/keendelivery-woocommerce-koppeling/class-keendelivery-orders.php:574
3.  KeenDelivery_Orders->add_column_content('track_trace')
    /wp-includes/class-wp-hook.php:289
4.  WP_Hook->apply_filters('', Array(2))
    /wp-includes/class-wp-hook.php:311
5.  WP_Hook->do_action(Array(2))
    /wp-includes/plugin.php:478
6.  do_action('manage_shop_order_post...', 'track_trace', 107327)
    /wp-admin/includes/class-wp-posts-list-table.php:1271
7.  WP_Posts_List_Table->column_default(WP_Post, 'track_trace')
    /wp-admin/includes/class-wp-list-table.php:1410
8.  WP_List_Table->single_row_columns(WP_Post)
    /wp-admin/includes/class-wp-posts-list-table.php:1304
9.  WP_Posts_List_Table->single_row(WP_Post, 0)
    /wp-admin/includes/class-wp-posts-list-table.php:751
10. WP_Posts_List_Table->_display_rows(Array(14), 0)
    /wp-admin/includes/class-wp-posts-list-table.php:728
11. WP_Posts_List_Table->display_rows()
    /wp-admin/includes/class-wp-list-table.php:1322
12. WP_List_Table->display_rows_or_placeholder()
    /wp-admin/includes/class-wp-list-table.php:1249
13. WP_List_Table->display()
    /wp-admin/edit.php:449

Context

URL            : /wp-admin/edit.php?s=xxxx&post_status=all&post_type=shop_order&m=0&layout=xxxxxxx&_customer_user&action=-1&paged=1&action2=-1
Referer        : /wp-admin/edit.php?post_type=shop_order
HTTP Method    : GET
HTTP Status    : 200
isSSL          : Yes
Current Filter : manage_shop_order_posts_custom_column
Execution Time : 4.382 s
Memory Usage   : 117.77 MiB
PHP SAPI       : fpm-fcgi