AttributeError: 'X' object has no attribute '_table_fieldnames'

After I created a new docType called Shipper Order, I navigated to the Shipper Order Form to check if it was properly rendered. and it was rendered successfully, but upon reloading the form page, an error appeared. and I’ve not made any change to the Shipper Order Controller

AttributeError: ‘ShipperOrder’ object has no attribute ‘_table_fieldnames’

How can i solved this error? if create new docType but without the Virtual enable all working fine.