Crash when trying to print in serial printer

[jankoJelic]

Hello, I have reasons to believe this issue is specific for serial printers, because the same code has been working perfectly for all other printers up until now.

Steps to reproduce:

1. I import USBPrinter and run init() method
2. I successfully find devices with getDeviceList method
3. I successfully connect to printer and confirm the access in the Android alert modal
4. I run any printing method (printBill, printText, printRaw...)
5. App crashes with this java error:
   Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.hardware.usb.UsbDeviceConnection.bulkTransfer(android.hardware.usb.UsbEndpoint, byte[], int, int)' on a null object reference
   at com.pinmi.react.printer.adapter.USBPrinterAdapter$2.run(USBPrinterAdapter.java:235)
   at java.lang.Thread.run(Thread.java:919)

I have been stuck at this one for a while now, do you have any tips this type of error?