Acess_token null in Google Drive Api

I am trying to update a file in Google Drive using Javascript, but the acess_token returns null. I based my file in this question Google Drive API V3 (javascript) update file contents.

const update = () => {
    gapi.load('client:auth2', () =>{
      gapi.client.init({
        apiKey: 'MY API KEY',
        clientId: '**************.apps.googleusercontent.com',
        discoveryDocs: ["https://www.googleapis.com/discovery/v1/apis/drive/v3/rest"],
        scope: 'https://www.googleapis.com/auth/drive',
      }).then(v => {
        var content = 'Hello World';
        var contentBlob = new  Blob([content], {
          'type': 'text/plain'
        });
        updateFileContent('File id', contentBlob, function(response) {
          console.log(response);
        });
      })
    })


  const updateFileContent = (fileId, contentBlob, callback) => {
    var xhr = new XMLHttpRequest();
    xhr.responseType = 'json';
    xhr.onreadystatechange = function() {
      if (xhr.readyState != XMLHttpRequest.DONE) {
        return;
      }
      callback(xhr.response);
    };
    xhr.open('PATCH', 'https://www.googleapis.com/upload/drive/v3/files/' + fileId + '?uploadType=media');
    xhr.setRequestHeader('Authorization', 'Bearer ' + gapi.auth.getToken().access_token);
    xhr.send(contentBlob);
  }

Someone know what Im doing wrong?