Undefined offset

I get this error message on the login screen:

Notice: Undefined offset: 1 in /home/imaginas/public_html/wp-content/plugins/hc-custom-wp-admin-url/hc-custom-wp-admin-url.php on line 98

How can I fix this? Thanks.

https://ww.wp.xz.cn/plugins/hc-custom-wp-admin-url/