Javascript Promise Chaining Issues (Ember)

I'm trying to upload a file to a remote server, get the path for that file on that server, and then save it in a record to my own API. Over half the time, the code crashes because returnedNewFile is undefined (though I've checked console.log(newFile) in the previous .then() function and it's always a promise object. What could be causing this behavior? What could be causing returnedNewFile to sometimes be null/undefined? I'm using Django REST + django-rest-json-api + ember.js 2.6, and this code is in a component's javascript file. Thank you in advance!

  store.findRecord('file', folderid).then(function(folder) {
    var file = fileList.pop();
    var newFile = fm.uploadFile(folder, file.name, file);
    return newFile;

  }).then(function(returnedNewFile) {
    var name = returnedNewFile.get('name');
    var path = returnedNewFile.get('path');
    var doc = store.createRecord('document', {
      name: name,
      path: path,
    });
    console.log("doc: ", doc);
    doc.save();
    return doc;

  }).then(function(doc) {
    console.log("hi number three");
    var grant = store.createRecord('grant', {
      department: department,
      number: number,
      document: doc,

    });
    grant.save();
    return grant;

  }).then(null, function(error) {
    console.log("Oops: " + error.message)
  });