How to check for null or emptystring for my Combo Box value

I getting an error code because the Combox value is null. How to fix
AddIntSettingInfoWithStringControl(Shared.Settings.Customer.ApplicationSettings.Global.FormToShowLTLDispatch, () => FormToShowLTLDispatchComboBox.SelectedValue, v => FormToShowLTLDispatchComboBox.SelectedValue = v.ToString(CultureInfo.InvariantCulture), () => (!DisableShipmentDetailFormRadioButton.Checked && !IntegrateShipmentDetailSsrsRadioButton.Checked && !IntegrateShipmentDetailRadioButton.Checked && !IntegrateLTLDispatchSsrsRadioButton.Checked)); Getting error because I have no value in the combobox