update gtree with alternative offspring function name doesn't work?

[ghost]

library(gWidgets2) ##GUI
library(gWidgets2RGtk2)
options(guiToolkit = "RGtk2")

Consider

NewWindow <- gwindow(title="Test", width=800, height=600)
gt <- gtree(offspring=gtreeoffspring,chosen.col="Path", offspring.col="isfolder", tooltip.col="icontype", visible=TRUE, container=NewWindow)

Window with gtree works and offspring successfully displayed
However with update

> update(gt)

Returns

Error in offspring(c(), offspring_data) : 
  could not find function "offspring"

If the function 'gtreeoffspring' is called 'offspring' instead, the function succeeds
Therefore seems to be a problem with gtree custom offspring function names