With WordPress Debug Bar, I got a lot of warning messages when I activate your plugin.
Notice: Undefined index: page in /wp-content/plugins/newsletter/plugin.php on line 285, 286, 426 and 438.
In addition, I got an alert saying:
The plugin generated 21435 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
I tried yet deactivating all others plugins and to turn only Newsletter on, but it throws me the same messages.