I'm trying to move items back and forth between a listbox and combobox, and this is what I have so far. It works fine to move items from the combobox to the listbox, but I get a System.ArgumentNullException error when I try to go the other direction. Any help would be appreciated

private void stateListBox_SelectedIndexChanged(object sender, System.EventArgs e

stateComboBox.Items.Add ( stateListBox.SelectedItem )
stateListBox.Items.Remove ( stateListBox.SelectedItem )


private void stateComboBox_SelectedIndexChanged(object sender, System.EventArgs e

stateListBox.Items.Add ( stateComboBox.SelectedItem )
stateComboBox.Items.Remove ( stateComboBox.SelectedItem )

}