null object reference when calling a method from an extended activity

Question

i am making a bluetooth service in android studio. i execute the service in a single activity successfully using some post in stackOverFlow like : How to move Bluetooth activity into a Service but when i want to send data from a second activity that extends first activity, to the bluetooth device i got a "null object reference".

public void onClick(View v) {
            sendToDevice(editSendData.getText().toString());
        }

sendToDevice is a non-private method in MainActivity(first Activity):

public void sendToDevice(String inMain){

    printerService.sendingData(inMain);
    Log.i("3","thread id:\n"+"send1");
}

//printerService is bluetoothService

//sendData is a method in bluetoothService that uses bytes of "inMain" to send data using outputStream

i do not know why but as i know, when second activity extends first activity(an activity is class), it also extends non-private methods of first activity. so why this error occurs?

i did not find a question like this one but there was a question in the comment of another question simillar to my question but that wasn't answered.

thank you a lot.

any help would be appreciated.

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null object reference when calling a method from an extended activity

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