ZebraPrinterLinkOs always null

Hello,

Can you help me please,

I want  upload my fonts from android device to the printer memory.

in the code below, object ZebraPrinterLinkOs  always null . Where is my problem?(I'm using printer zd410)

Connection connection = new BluetoothLeConnection(macAddress, this);

ZebraPrinter printer = ZebraPrinterFactory.getInstance(PrinterLanguage.ZPL, connection);

Thread.sleep(10000);

ZebraPrinterLinkOs linkOsPrinter = ZebraPrinterFactory.createLinkOsPrinter(printer, PrinterLanguage.ZPL);

linkOsPrinter.downloadTtfFont("/storage/emulated/0/fonts/DroidItalic.ttf", "Z:");  // exception, linkOsPrinter is null

Thank you in advance