'Return a GrrMessage instance from the transaction log or None.'

def GetTransactionLog(self):

    try:
        (value, reg_type) = _winreg.QueryValueEx(self._GetKey(), 'Transaction')
    except exceptions.WindowsError:
        return
    if (reg_type != _winreg.REG_BINARY):
        return
    try:
        return rdf_flows.GrrMessage.FromSerializedString(value)
    except message.Error:
        return
