Error messages when posting comments

On several occasions, when a reader posts a comment, they receive this error message

Warning: Undefined variable $comment in /home/westvall/public_html/wp-content/plugins/subscribe-to-comments/subscribe-to-comments.phponline504
Warning: Attempt to read property “comment_approved” on null in /home/westvall/public_html/wp-content/plugins/subscribe-to-comments/subscribe-to-comments.phponline504
Warning: Undefined array key “subscribe” in /home/westvall/public_html/w-content/plugins/subscribe-to-comments/subscribe-to-comments.phponline504
Warning: Cannot modify header information – headers already sent by (output started at /home/westvall/public_html/wp-content/plugins/subscribe-to-comments/subscribe-to-comments.php:504)in /home/westvall/public_html/w-includes/comment.phpon line 557
Warning: Cannot modify header information – headers already sent by (output started at /home/westvall/public_html/wp-content/plugins/subscribe-to-comments/subscribe-to-comments.php:504)in /home/westvall/public_html/w-includes/comment.phpon line 558
Warning: Cannot modify header information – headers already sent by (output started at /home/westvall/public_html/wp-content/plugins/subscribe-to-comments/subscribe-to-comments.php:504)in /home/westvall/public_html/w-includes/comment.php on line 559
Warning: Cannot modify header information – headers already sent by (output started at /home/westvall/public_html/w-content/plugins/subscribe-to-comments/subscribe-to-comments.php:504)in /home/westvall/public_html/w-includes/pluggable.php on line 1427
Warning: Cannot modifv header information – headers already sent by (output started at /home/westvall/publichtml/w-content/plugins/subscribe-to-comments/subscribe-to-comments.php:504)in /home/westvall/public_html/w-includes/pluggable.phpon line 1430

This occurs when a reader posts a comment and also when I, logged in as administrator, post a reply. The comments and replies are posted, but it’s up to the reader to return to the page.

Thank you for any help you could provide.

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