title: wpf comboBox set selectedindex -1 but still show previous text
body: <p>First, i select one item on the comboBox when the application is running.</p>
  
  <p>Then i make some operations and after that i set the comboBox's selectedIndex -1 in the code. Actually, the comboBox's selectedItem is null now but it still displays the previous text on the UI. How can I fix that?</p>