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