i found a bug similar to forum post, but not the same.
In MultiColumnComboBox when SelectionBoxesVisibility is set to 'collapsed' and and whatever you set AutoCompleteMode to (different from the forum post),
if you select the first item it is not shown. Although the NullText is removed. It works perfectly for all the other items. This is quiet a problem if you only have one item in the dropwdown :D
The following error is thrown when the dropdown of the control is opened: Cannot find governing FrameworkElement or FrameworkContentElement for target element. ... ; target element is 'GridViewItemsSourceProvider; target property is 'ItemsSource' (type 'Object')
If the combobox is placed inside a RadGridView, the error is thrown each time the dropdown is opened (after a commit action).