Null Object Reference In PB12.6

Hi,

     In 11.5,the code below is ok.But in 12.6 build 3506,it makes a 'null object reference' error.

     'w_main_runner' is the window name , a main window opened with OpenSheetWithParmDocked in mdi frame.

           //--------------

           w_main_runner.wf_runprogram(lstr_runinfo)

          //-----------------

     Besides, Is there higher build than 3506?