Error message after install

Hi,

I got the following error message as soon as I activated the plugin:

Notice: Undefined index: newsletters in /web/kalamarbeat/beatakalamar.com/wp-content/plugins/plaintext-newsletter/plugin.php on line 14

Notice: Undefined index: newsletters in /web/kalamarbeat/beatakalamar.com/wp-content/plugins/plaintext-newsletter/plugin.php on line 34

Notice: Undefined index: transactional in /web/kalamarbeat/beatakalamar.com/wp-content/plugins/plaintext-newsletter/plugin.php on line 34

Warning: Cannot modify header information – headers already sent by (output started at /web/kalamarbeat/beatakalamar.com/wp-content/plugins/plaintext-newsletter/plugin.php:14) in /web/kalamarbeat/beatakalamar.com/wp-includes/functions.php on line 6828

Warning: Cannot modify header information – headers already sent by (output started at /web/kalamarbeat/beatakalamar.com/wp-content/plugins/plaintext-newsletter/plugin.php:14) in /web/kalamarbeat/beatakalamar.com/wp-admin/includes/misc.php on line 1310

Warning: Cannot modify header information – headers already sent by (output started at /web/kalamarbeat/beatakalamar.com/wp-content/plugins/plaintext-newsletter/plugin.php:14) in /web/kalamarbeat/beatakalamar.com/wp-admin/admin-header.php on line 9

can someone pls help?