对象未设置为对象的实例!! ComboBox SelectedIndex SelectedItem

I am developing a program which gets all Clearcase regions (basically strings) & adds them to Combo box. I compare an existing clearcase region string in newly added items in combo box & if it is found then I want to select it, but because nothing is selected for the first time, selectedItem is null & selectedIndex = -1. when I assign 0 to selectedIndex, error comes --> object not set to instance of an object!! same problem when you assign something to selectedItem; gives an error.I am developing a program which gets all Clearc