Please refer to the attached sample project and run it. Select a random item and press the button. You will obtain the error. If no selection is available, the item is removed as expected. 

Workaround: use Begin/EndUpdate block while deleting a record from the DataTable:

        Me.RadGanttView1.BeginUpdate()
        _dataset.Tables("Tasks").Rows.RemoveAt(2)
        Me.RadGanttView1.EndUpdate()