add null check

app/src/main/java/xyz/mtfos/btdemo/PrinterBle.java

@@ -312,7 +312,7 @@ public class PrinterBle {
     }
 
     private void sendData(byte[] data) {
-        if (data == null || data.length < 1 || data.length > 20) return;
+        if (data == null || data.length < 1 || data.length > 20 || printerCharacteristic == null) return;
 
         boolean setVal = printerCharacteristic.setValue(data);
         boolean sendVal = mBtGatt.writeCharacteristic(printerCharacteristic);