illegal string offset length

Hi after activation I got this error and notice

Warning: Illegal string offset ‘length’ in /xxxxxxxx/wp-content/plugins/children-excerpt-shortcode-plugin/children-excerpt.php on line 38

Notice: Uninitialized string offset: 0 in xxxxx/wp-content/plugins/children-excerpt-shortcode-plugin/children-excerpt.php on line 38

https://ww.wp.xz.cn/plugins/children-excerpt-shortcode-plugin/