One more thing. Don't use a combo unless you want to allow the user to input data. (If you do, then the data are available in the text property.)
by far the more common use of the combo box is the dropdown list style, which actually isn't a combo box at all.
A little history here: there are two controls, listbox and combobox. Listbox means that the user can't input data, combobox means the user can. Simple means an area on the screen that contains a scrollable list, dropdown means a single list item and a dropdown arrow to show more of the list when clicked.
Listbox predates combobox, and doesn't include a dropdown type. So, rather than change the listbox control to allow simple or dropdown, and have the combobox be a separate control, the dropdown listbox type (with no ability for user input) was rolled into the combobox control and its style property.
I gave you all this because it seemed to me that you're confusing all of this when asking about null values. If you have a list box, as opposed to a combo, there aren't any null values in the list. Why would you, the developer, put null values in the list, as Tracy quite rightly asks? However, the listbox could be in pristine, never had a value selected, state. In this case, the listindex property is -1.
Now, on the other hand, you're also looking at the boundtext property, which suggests that you have bound the control to an outside data source, which itself appears to contain null values. If this is so, perhaps you could change your datasource to exclude null records. If you want to know how to do this, provide more detail about how you're binding your listbox.
HTH
Bob