混淆报错，请问我该怎么处理

[aa563011992]

W/IModbusWorker: ModbusMaster init failed
com.serotonin.modbus4j.exception.ModbusInitException: java.lang.NullPointerException
at com.serotonin.modbus4j.serial.SerialMaster.init(SourceFile:84)
at com.serotonin.modbus4j.serial.rtu.RtuMaster.init(SourceFile:71)
at com.licheedev.modbus4android.ModbusWorker$5.call(SourceFile:299)
at com.licheedev.modbus4android.ModbusWorker$5.call(SourceFile:281)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:841)
Caused by: java.lang.NullPointerException
at java.io.File.fixSlashes(File.java:185)
at java.io.File.(File.java:134)
at android.serialport.SerialPort$Builder.(SourceFile:147)
at android.serialport.SerialPort$Builder.(SourceFile:132)
at android.serialport.SerialPort.newBuilder(SourceFile:129)
at com.licheedev.modbus4android.AndroidSerialPortWrapper.open(SourceFile:72)
at com.serotonin.modbus4j.serial.SerialMaster.init(SourceFile:75)
at com.serotonin.modbus4j.serial.rtu.RtuMaster.init(SourceFile:71) 
at com.licheedev.modbus4android.ModbusWorker$5.call(SourceFile:299) 
at com.licheedev.modbus4android.ModbusWorker$5.call(SourceFile:281) 
at java.util.concurrent.FutureTask.run(FutureTask.java:237) 
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112) 
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587) 
at java.lang.Thread.run(Thread.java:841) 

#modbus
-keep class com.licheedev.** { ; }
-keep class com.serotonin.* { ; }
-keep class android.serialport.* { *; }