No access possible via a 'NULL' data reference

Hello Experts,

I have created Thing Based Screens, For the BO,Which is Contains some Root Elements along with the one Sub Node, In QC Screen,I have added Advanced List Pane and binded to Sub Node elements and I have Added a button Called "Add to List" in QC screen to Create a new Row for Advanced List pane ,that button will trigger "Add Row" event of the List . When I Click on that button , Its throwing an dump, When I checked in Dump Analysis window, Its showing an Error like No access possible via a 'NULL' data reference. Previously I have Done same thing for others its working properly, I don't Know why this is not working, Please tell me how to resolve......

And also Please Tell me , in Standard Sales Order QC screen , Once the first row is filled in Advanced list pane (Item Level), Another Row will be opened automatically , How to do this for Custom ,

Thanks in Advance

Thanks and Regards

Ravi