A problem with wialon.js

/// Fetch varable from 'GET' request
let get_url_parameter = function (name, def) {
  if (!name) {
    return null;
  }
  let pairs = decodeURIComponent(document.location.search.substr(1)).split("&");
  for (let i = 0; i < pairs.length; i++) {
    let pair = pairs[i].split("=");
    if (pair[0] === name) {
      pair.splice(0, 1);
      return pair.join("=");
    }
  }
  return def || null;
};

/// Load scripts
function load_script(src, callback) {
  let script = document.createElement("script");
  script.setAttribute("type","text/javascript");
  script.setAttribute("charset","UTF-8");
  script.setAttribute("src", src);
  if (callback && typeof callback === "function") {    
      script.onload = callback;    
  }
  document.getElementsByTagName("head")[0].appendChild(script);
}[/i]

[i]let ready = (callback) => {
if (document.readyState != "loading") callback();
else document.addEventListener("DOMContentLoaded", callback);
}[/i]

[i]//DOM is ready
ready(() => {
  let branch = get_url_parameter("b","master");
  let url = get_url_parameter("baseUrl",(branch=="develop"?"https://dev-api.wialon.com":"https://hst-api.wialon.com"));
  if (!url)
    url = get_url_parameter("hostUrl","https://hosting.wialon.com");
  if (!url)
    return null;
  url += "/wsdk/script/wialon.js";  
  // load wialon sdk
  load_script(url, function() {
    let session = wialon.core.Session.getInstance();
    session.initSession('https://hst-api.wialon.com');
  });
});