Undefined property: stdClass::$thumb

I get a number of these PHP Notices in debug.log:

Undefined property: stdClass::$thumb in .../wp-content/plugins/tag-or-category-term-group-order/lib/tctgo_plugin.class.php on line 47

Any idea why this occurs?

http://ww.wp.xz.cn/extend/plugins/tag-or-category-term-group-order/