When a new record it added, the business object automatically initializes the values within the new row to remove any DBNull.Value values. To prevent this, and allow all of the fields within the new row to be NULL, set the AllowNullValuesOnNewRow property on the business object to True.If you only want a few fields to have NULL values, then in the SetDefaultValues event handler, add this code:
Me.CurrentRow("fieldname") = DBNull.Value
Add that for each field you want to initialize as a null value, and you'll be good to go.