Site Error When Adding External Links to Main Navigation Drupal 8.4

I get the following error if I add a external link to the main navigation. i.e. https://www.example.com

"The website encountered an unexpected error. Please try again later."

The error log shows the following error.

Type: php
Location: /node
Referrer: /admin/structure/menu/manage/main
Message: UnexpectedValueException: External URLs do not have an internal route name. in Drupal\Core\Url->getRouteName() (line 555 of /core/lib/Drupal/Core/Url.php).
Severity: Error

Notes:

- Drupal 8.4: External menu links work with Bartik and Bootstrap themes, broken in AT.
- Drupal 8.2.8 External menu links work fine with AT

Site Info

Drupal: 8.4.0
Web Server: Apache/2.4.17 (Unix) OpenSSL/1.0.1h mod_fcgid/2.3.9
PHP Version: 7.0.14
Memory limit: 256M
Database Version: MySQL