Error thrown when setting cdn to "None - compile locally"

When setting cdn to None - compile locally I get the following exceptions being thrown in bootstrap or bootstrap sub-theme:

Exception has occurred.
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. Valid plugin IDs for Drupal\bootstrap\Plugin\ProviderManager are: jsdelivr, cdnjs, _broken, drupal_bootstrap_styles, custom

This is called from CdnProviderBase->alterForm when setting $this->provider e.g

$this->provider = ProviderManager::load($this->theme, $provider);