DataView Not found in RowChanged event

So I have a OrderDtl Dataview and , an event is created with a RowChanged trigger.
What I want is to have a Separate Textbox which will be update from the event.
The TextBox is bound to OrderHed.PartNumberTest .

Attached image is the Row update widget properties.
So it updates when the row is changed for the first time. and when I select the next row, It shows an error OrderDtl not found

N.B. : I am very new to the Epicor customization, So any help is appreciated