Hi Rishi,
You must use the adapter's overload that accepts an IPersistenceInfo parameter, you just pass Nothing to it:
Dim sorter As New SortExpression()
sorter.Add(New SortClause(EmpFormFields.LastUpdatedTimestamp, Nothing, SortOperator.Descending))
You also can use this version:
Dim sorter As New SortExpression()
sorter.Add(New SortClause(EmpFormFields.LastUpdatedTimestamp Or SortOperator.Descending))
This is described in detail in the Adapter - Sorting manual's section.
Hope helpful