Issue resolved with this patch https://nimb.ws/gFA06i

Issue resolved with this patch https://nimb.ws/gFA06i

Here is the code in non-image format:

    /* set customer email if email is found empty (bug) */
    if ($this->_quote->getCustomerEmail() === null && $this->_quote->getBillingAddress()->getEmail() !== null) {
        $this->_quote->setCustomerEmail($this->_quote->getBillingAddress()->getEmail());
    }