The following code implements a hook to _screen for the destroy event:
To test it, you just have to run the code then issue the following command:

_Screen.Destroy

If you issue it again, a wait window will confirm the object is no longer there. Why then does _Screen still access the hooked method and how do I un hook it? (Notice that an attempt is made in the code but this does not work)

Thanks to anyone who can help

_SCREEN.AddObject('oHook', 'ScreenHook')

DEFINE CLASS ScreenHook AS Custom
  oScreen = _Screen

  PROCEDURE INIT
    This.oScreen = _Screen
  ENDPROC

  PROCEDURE Destroy
     WAIT WINDOW 'Destroy Hook'
     This.oScreen = NULL
  ENDPROC

  PROCEDURE oScreen.destroy
    IF TYPE('This.oHook') = 'O'
      IF VARTYPE(This.oHook) = 'O'
        WAIT WINDOW 'oHook exists'
        This.RemoveObject('oHook')
      ENDIF
    ENDIF
    WAIT WINDOW '_Screen Destroy'
  ENDPROC
ENDDEFINE