My custom module is breaking the Calendar with "Error while saving"

Hi,

I created a custom module with a manifest and a after_ui_footer logic hook.

Each time I install my module it's working fine, but the calendar is throwing "Error while saving".

I've tried a couple of things but I don't know why I have this message and I need help.

If I remove my module, I don't have the error message. Same with Sugar 6 and 7.

Code of my class and function fired with an after_ui_footer logic hook:

<?php
if (!defined('sugarEntry') || !sugarEntry) die('Not A Valid Entry Point');
class Calendar_style_logic_hook_class
{
  function Calendar_style_logic_hook_method($event, $arguments)
  {
    if (!isset($_REQUEST["to_pdf"]) || $_REQUEST["to_pdf"] == false)
    {
        if($_REQUEST['module'] == 'Calendar')
        {
            echo '<link rel="stylesheet" href="custom/modules/cal.css"/>';
        }
    }
  }
}

Thanks,

Jonathan