Problems with javascript function onchange

Hello stackoverflow people, i've been coding for sometime and got stuck with this script. Because i cant find out why im getting console error:

TypeError: document.getElementById(...) is null


var Nlang = document.getElementById('langSelect').value;

ive tried doing as function onchange and ve tried this:

document.getElementById('langSelect').onchange = function() {

But that didnt worked too. What is wrong with this script, here is my javascript code:

function WasChanged(){
    var Nlang = document.getElementById('langSelect').value;
    var currentUser = <?php echo json_encode($AppUI->user_id); ?>;
};

And here is my php code:

    echo arraySelect($langlist, 'pref_name[LOCALE]', 'onchange="WasChanged()" class="text" size="1"', $userLang, true);