Currently, when DataContext of RadMultiColumnComboBox changes to an object that contains valid SelectedItem and ItemsSource view model properties, the SelectedItem get set to null. This happens because the control resets the selection on ItemsSource changed, which happens after the SelectedItem binding was evaluated. 

Add an option to ensure that the SelectedItem won't get reset after the binding was evaluated  because of the ItemsSource changed action. The behavior should mimic how RadComboBox behaves in this scenario.