Can access DOM ElementByID

Question

I have a js file:

I am trying to read the value f a dropdown list which has an ID = "topn". When I do that at POS1 it gives an error :

Cannot read property 'options' of null

At POS2 it works fine. But I want the value at POS1. Where have I gone wrong??. I am using latest version of Google Chrome.

window.onload = getAll();

function getAll() {
  var data = new FormData();
  //console.log(document.location.href);

  /* POS1 */
  var test = document.getElementById("topn");
  console.log(test.options[test.selectedIndex].value);
  /**/

  data.append('top_n', test.options[test.selectedIndex].value);
  data.append('top_n', 5);

  var xhr = new XMLHttpRequest();
  xhr.open('POST', 'http://xyz.php', true);
  xhr.onload = function() {
    console.log("done here");
    var table_dom = document.getElementById("tab1");
    /* POS2 */
    var test = document.getElementById("topn");
    console.log(test.options[test.selectedIndex].value);
    /**/
    response_text = this.responseText;
    response_text.replace(/['"]+/g, '');
    console.log(response_text);
    table_dom.innerHTML = response_text;
  };
  xhr.send(data);
}