Error accessing the Bootstrap Settings page

Hi,

I know version 4.x it's still in an early stage, but just to notify, I've just found the following issue deploying the current dev version of Bootstrap 4.x.

I can browse the admin and content pages but the admin/appearance/settings/bootstrap page which display the following message in the browser and the apache error:

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

and I had the following error in my web server log:

Uncaught PHP Exception Drupal\\Core\\Security\\UntrustedCallbackException: "Render #pre_render callbacks must be methods of a class that implements \\Drupal\\Core\\Security\\TrustedCallbackInterface or be an anonymous function. The callback was Drupal\\bootstrap\\Plugin\\Prerender\\Link::preRender. See https://www.drupal.org/node/2966725" at SITEPATH/web/core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php line 96, referer: http://site/admin/appearance