SilverStripe custom admin base url fix for ReportAdmin

config.yml

ReportAdmin:
  extensions:
    - 'ReportAdminExtension'

ReportAdminExtension.php

<?php


class ReportAdminExtension extends Extension {

    public function updateEditForm($form) {
        $gridField = $form->Fields()->dataFieldByName('Reports');
        if ($gridField instanceof GridField) {
            $columns = $gridField->getConfig()->getComponentByType('GridFieldDataColumns');
            $columns->setFieldFormatting(array(
                'title' => function($value, &$item) {
                    $link = Controller::join_links(
                        Config::inst()->get('AdminRootController', 'url_base'),
                        Config::inst()->get('ReportAdmin', 'url_segment'),
                        get_class($item)
                    );

                    return sprintf(
                        '<a href="%s" class="cms-panel-link">%s</a>',
                        Convert::raw2xml($link),
                        Convert::raw2xml($value)
                    );
                }
            ));
        }

    }

}