Error when first viewing the form

An error comes up when viewing the config form before saving any changes:

Warning: Invalid argument supplied for foreach() in Drupal\Core\Render\Element\Checkboxes::valueCallback() (line 100 of /core/lib/Drupal/Core/Render/Element/Checkboxes.php)

This is because there's no check to see if a config value exists before trying to use it.