Undefined index

Great plugin! It works wonderfully, but it shows this notice on the login page:

Notice: Undefined index: redirect_to in /home/xxxxxxxxxx/public_html/auction/wp-content/plugins/sky-login-redirect/sky-login-redirect.php on line 20

Users can login without problem, and they are redirected to the page they were browsing previously, but we’re concerned about the notice. How can prevent that from showing up?Is it a conflict with the WordPress install or another plugin?

The site we’re using it in is http://crowderauctions.com/

We would greatly appreciate your help.

https://wordpress.org/plugins/sky-login-redirect/