Hi Alfia,
I get the same error, but I simply ignore it and continue working on the XDP, I have found useful to link the XDP with an XML set of data rather than using the test xml data, to do so I go to the Output history of a document (Sales Order, Purchase Order, etc.) and "save as" XML, then link it via the "Form Properties" item on the "file menu".
Hope it helps,
Marcos