Error message on re-check

If I try to send (or resend) a test without reload the page, I mean…. just changing the destinatary I get this message on the top of the page:

Notice: Undefined index: checkemail_break in /var/.../wp-content/plugins/check-email/check-email.php on line 182

And the test message is not delivered!

https://www.ads-software.com/plugins/check-email/