Listening to uimenu ChildAdded event causes screen print

Does anybody know why the following code:

m = uimenu(figure,'Text','ParentMenu');
lsn = addlistener(m,'ChildAdded',@(~,~) []);
m2 = uimenu(m,'Text','ChildMenu');

produces the following undesired screen print?

ui::eventdata::ChildData::ChildData()

More importantly, since it doesn't appear to be an actual warning, is there any way to turn it off? The uimenu code is built-in, so I can't peek at the classdef file to see if it is a rogue disp call in the event definition.

Bonus points, but not the primary question: does anybody know how to trigger the 'NodeChildAdded' event?