Steps to reproduce the issue

1. open com_redirect
2. in notice link click on 'Redirect System Plugin'
   ... modal with redirect plugin is opened
3. click on the close button (or on the close X) in the modal

Expected result

plg_system_redirect should be checked in again

Actual result

plg_system_redirect is not checked in, but still checked out.

System information (as much as possible)

PHP Built On Linux ruud-development 4.10.0-35-generic #39-Ubuntu SMP Wed Sep 13 07:46:59 UTC 2017 x86_64
Database Version 5.7.19-0ubuntu0.17.04.1
Database Collation utf8_general_ci
Database Connection Collation utf8mb4_general_ci
PHP Version 7.0.24-1+ubuntu17.04.1+deb.sury.org+1
Web Server Apache/2.4.25 (Ubuntu)
WebServer to PHP Interface apache2handler
Joomla! Version Joomla! 3.8.1 Stable [ Amani ] 4-October-2017 14:00 GMT
Joomla! Platform Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:56.0) Gecko/20100101 Firefox/56.0

Additional comments

the close button can be 'fixed' by replacing the following code in ./administrator/components/com_redirect/views/links/tmpl/default.php
find

'<button class="btn" data-dismiss="modal" aria-hidden="true">'
						. JText::_("JLIB_HTML_BEHAVIOR_CLOSE") . '</button>'

replace with

'<button type="button" class="btn" data-dismiss="modal" aria-hidden="true"'
							. ' onclick="jQuery(\'#plugin' . $this->redirectPluginId . 'Modal iframe\').contents().find(\'#closeBtn\').click();">'
							. JText::_('JLIB_HTML_BEHAVIOR_CLOSE') . '</button>'

The behavior of the X button just bypasses all close logic for the plugin and 'hard' closes the modal.
Code for this is (or should be?) in ./administrator/components/com_plugins/views/plugin/tmpl/modal.php

I have unfortunately no clue how to fix that :(