Categories app error

I just updated to Perch 2.8.19 from 2.8.18 and the blog home page is displaying the PHP error message "Undefined index: catDepth". The blog was working perfectly before the update.

As a temporary measure, I have hacked the perch/core/apps/categories/PerchCategories_Category.class.php as below:

    public function to_array()
    {
        $r = parent::to_array();

        if (isset($this->details['catDepth'])) {
            $r['catDepth'] = $this->details['catDepth'];
        }

        return $r;
    }

Diagnostic report: Diagnostics report HEALTH CHECK

Perch is up to date PHP 7.0.0-5+deb.sury.org~trusty+1 is up to date MySQL 5.7.9 is up to date Image processing available File upload size is low. You can only upload files up to 2M. SUMMARY INFORMATION

Perch: 2.8.19, PHP: 7.0.0-5+deb.sury.org~trusty+1, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: 7e72f9690b1498a1bead7a637c33a831c0d2f655 $, with PDO Server OS: Linux, fpm-fcgi Installed apps: content (2.8.19), assets (2.8.19), categories (2.8.19), perch_blog (4.6) App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', ); PERCH_LOGINPATH: /perch PERCH_PATH: /home/forge/isisps.co.uk/public/perch PERCH_CORE: /home/forge/isisps.co.uk/public/perch/core PERCH_RESFILEPATH: /home/forge/isisps.co.uk/public/perch/resources Image manipulation: GD PHP limits: Max upload 2M, Max POST 8M, Memory: 512M, Total max file upload: 2M Resource folder writeable: Yes HTTP_HOST: isisps.co.uk DOCUMENT_ROOT: /home/forge/isisps.co.uk/public REQUEST_URI: /perch/core/settings/diagnostics/ SCRIPT_NAME: /perch/core/settings/diagnostics/index.php

Regards, Chris