private void enableListening(IScanAddOns isao, boolean listen) {
if (isao instanceof ScanAddOnModel) { // this test returns false if isao is null
ScanAddOnModel saom = (ScanAddOnModel) isao;
if (listen) {
saom.addListener(listenerScanAddOn);
}
else {
saom.removeListener(listenerScanAddOn);
}