как исправить ошибку × TypeError: Cannot read property 'firstChild' of null

 <text name="text1" id="text1">asxacasc</text>
 <Button size="xl" level="2" onClick={tern()}>
            ОТПРАВИТЬ ЗАЯВКУ
                </Button>
    </Panel>
);
function tern() {
    try {
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = function () {

            if (this.readyState == 4 && this.status == 200) {
                var el = document.getElementById("text1");
                el.firstChild.data = "Unlock";
            }
            else {
                var el = document.getElementById("text1");
                el.firstChild.data = "lock";
            }


        };
    }
    catch
    {
    }
    xmlhttp.open("POST", "https://cvetyservervk.000webhostapp.com/test.php", true);
    xmlhttp.send({ method: "proverka", addres: "asdasd" });

}
Persik.propTypes = {
    id: PropTypes.string.isRequired,
    go: PropTypes.func.isRequired,
};

export default Pro;