Should fix notifications "sticking" when closing already-removed ones

Author: RedEnchilada

plugins/Notification/removenotifications.php

@@ -88,6 +88,7 @@ function handle($args)
 
 		foreach($notes as $note) {
 			$item = User_notification::staticGet('id', $note);
+			if (!$item) continue;
 			if($user->id != $item->user_id) {
 				$this->clientError(_('That notification isn\'t yours!'));
 				return false;