Exception #0 (Zend_Db_Statement_Exception): SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`magento`.`newsletter_subscriber`, CONSTRAINT `NEWSLETTER_SUBSCRIBER_FIRSTNAME_CUSTOMER_ENTITY_FIRSTNAME` FOREIGN KEY (`firstname`) REFERENCES `customer_entity` (`firstname`) ON DELETE SET NULL), query was: INSERT INTO `newsletter_subscriber` (`store_id`, `change_status_at`, `subscriber_email`, `subscriber_status`, `firstname`, `firstname`) VALUES (?, '2019-02-12 15:35:20', ?, ?, ?, ?)

$con->addForeignKey(
                $setup->getFkName('newsletter_segment', 'firstname', 'customer_entity', 'firstname'),
                'newsletter_subscriber',
                'firstname',
                'customer_entity',
                'firstname',
                MagentoFrameworkDBDdlTable::ACTION_SET_NULL
            );

<field name="firstname">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="visible" xsi:type="boolean">true</item>
                    <item name="dataType" xsi:type="string">text</item>
                    <item name="formElement" xsi:type="string">input</item>
                    <item name="source" xsi:type="string">subscriber</item>
                    <item name="label" xsi:type="string">Firstname</item>
                </item>
            </argument>
        </field>