To reproduce:
- add a RadMultiColumnCombo and bind it to fill with data. Use the following code:

this.radMultiColumnComboBox1.EditorControl.SelectionChanged+=EditorControl_SelectionChanged;

private void EditorControl_SelectionChanged(object sender, EventArgs e)
{
    radMultiColumnComboBox1.SelectedIndex = -1;
}

After selecting a new row from the drop down grid, NullReferenceException is thrown.