The hook 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 MUIA_Popobject_Object, it's your task to tell when your popup is finished. You can terminate popups at anytime by sending a MUIM_Popstring_Close method:
A double click terminates the popping list with a successful return value.
DoMethod(plist,MUIM_Notify,MUIA_Listview_DoubleClick,TRUE, pop,2,MUIM_Popstring_Close,TRUE);