code: @Nonnull
    @Override
    public Map<String, Object> getData() {
      Map<String, Object> result = super.getData();
      Preconditions.checkNotNull(result, "Data in a QueryDocumentSnapshot should be non-null");
      return result;
    }
docs: Returns the fields of the document as a Map. Field values will be converted to their native
  Java representation.
  
  @return The fields of the document as a Map.