Move mouse over a placed object then open up the debug console (key 0 for me, might be ~ for others, make sure you don't use the mouse to open it)
Local exec this (couldn't find better solution to instantly show the EH working):
(get3DENMouseOver select 1) addEventHandler ["AttributesChanged3DEN","[0] call BIS_fnc_3DENNotification"]
it should show a "scenario saved" notification if you change any attribute of the object. (move it around, open attributes, etc).
Now use any widget. The EH doesn't trigger.