confirmed_message_text issue in subscription.php line no 585

Hello,

Now i’m using this plugin version 4.7.6 but while click on confirmation link at first time then error is occur.

Notice: Undefined index: confirmed_message_text in /home2/***/public_html/wp-content/plugins/newsletter/subscription/subscription.php on line 585

Warning: Cannot modify header information – headers already sent by (output started at /home2/***/public_html/wp-content/plugins/newsletter/subscription/subscription.php:585) in /home2/***/public_html/wp-content/plugins/newsletter/subscription/subscription.php on line 174

Warning: Cannot modify header information – headers already sent by (output started at /home2/***/public_html/wp-content/plugins/newsletter/subscription/subscription.php:585) in /home2/***/public_html/wp-content/plugins/newsletter/subscription/subscription.php on line 763

Thank you