When a popup is closed, this hook is called.
Source position: MUIClass.PopString.pas line 87
published property TMUIPopObject.OnObjStr : TNotifyEvent
You can examine the state of your PopObject and set the contents of the string gadget respectively.
The event will only be called when your popup is closed with a success value of True. Otherwise, MUI closes the popup without taking further actions, just as if had never opened.
Since MUI doesn't know anything about your PopObject, it's your task to tell when your popup is finished. You can terminate popups at anytime by sending a MUIM_Popstring_Close method.