"Unable to cast object of type..." error

Hi, 

I need to add a smart panel when clicked on my Add Purchase button. The smart panel should include all the Purchase Orders inside a grid which are currently available in the database. So I created a graph extension of my parent Graph APProformaEntry and also a DAC extension of px.objects.po.POOrder standard DAC called POOrderExtension to define new fields if I need to add. But for now I didn’t add any new fields to the DAC Extension except the main key .

below screenshots show the code for the reference.

In the customization project editor, inside the dialogs, I added a pop-up panel and passed POrdersView which is previously defined view in the graph extension , as the Key. Now the issue is when I preview my changes before publish, it shows me the following error.

I have no idea about this error, even I have not defined any new fields or changed the data type of the main key OrderNbr,  in my DAC extension, this error is happening continuously. Have I done something wrong in the code.  can someone help to solve this?

Thanks.