Exception on port already open

[practicalusage]

I get this exception when I do cen-gnd or power on off the device:

Going to open port: COM3.
Serial port open!
Getting bus... (now, please do reboot by CEN or by power off/on)
Getting bus failed, will try again - 0/100!
Got exception: System.UnauthorizedAccessException: Access to the port is denied.
at System.IO.Ports.InternalResources.WinIOError(Int32 errorCode, String str)
at System.IO.Ports.SerialStream.get_BytesToRead()
at System.IO.Ports.SerialPort.get_BytesToRead()
at BK7231Flasher.BK7231Flasher.Start_Cmd(Byte[] txbuf, Int32 rxLen, Single timeout)!
Exception caught: System.InvalidOperationException: The port is closed.
at System.IO.Ports.SerialPort.get_BytesToRead()
at BK7231Flasher.BK7231Flasher.consumePending()
at BK7231Flasher.BK7231Flasher.Start_Cmd(Byte[] txbuf, Int32 rxLen, Single timeout)
at BK7231Flasher.BK7231Flasher.linkCheck()
at BK7231Flasher.BK7231Flasher.getBus()
at BK7231Flasher.BK7231Flasher.doGetBusAndSetBaudRate()
at BK7231Flasher.BK7231Flasher.doGenericSetup()
at BK7231Flasher.BK7231Flasher.doReadInternal(Int32 startSector, Int32 sectors)
at BK7231Flasher.BK7231Flasher.doRead(Int32 startSector, Int32 sectors)
There was no result to save.