ParentWindow() return Null Object Reference In PB12.6

Hi,

     When I opened a sheet by 'OpenSheetwithParmAsDocument' ,the function ParentWindow() can not return the mdi frame object. But when I use 'OpenSheetwithParm', ParentWindow() can return the mdi frame object. Why?

     //OpenSheetwithParmAsDocument Code

     window lwobj

     OpenSheetwithParmAsDocument( lwobj, '', 'w_1', &

            w_main, String(Today(), "yy-m-d hh:mm")+"_"+ string(Rand(1000))  )

     return

     //