I migrated a magento 1 site to magento 2, and checkout (any theme) immediately failed with error:

Exception #0 (Magento\Framework\Exception\RuntimeException): Type Error occurred when creating object: Magento\Eav\Model\Entity\Attribute\Source\Config, Argument 1 passed to Magento\Eav\Model\Entity\Attribute\Source\Config::__construct() must be of the type array, null given

This post how I debugged/tracked down the issue.