To reproduce:
- Add several columns including combo-box columns to a grid. 
- Add one row and upon a button click, change the data source of the combo column.

Workaround: 

Seth the following field to null prior changing the data source: 

    GetType(GridViewComboBoxColumn).GetField("nullBoundItem", Reflection.BindingFlags.Instance Or Reflection.BindingFlags.NonPublic).SetValue(combo, Nothing)
    combo.DataSource = dt