I have fixed this problem, but I want to know why it happens.

Define myChkBox as ChkBox
  Caption = 'Re\<fused'      && User can press {alt+f} to toggle value
  Procedure When()
    local lo
    lo = Thisform.ActiveControl
    This.Value = !This.Value
    lo.SetFocus()
    Return .F.
  EndProd
EndDefine

This works just fine when the user clicks on the chkBox with the mouse, but if the user uses {alt+f} to do it, error "ACTIVECONTROL IS NOT AN OBJECT" and debugger shows "" for ActiveControl.

Why does the keyboard shortcut throw away the ActiveControl?