opc-ua

ENHANCEMENT

1. Describe the enhancement you are requesting.
   Consider using Go errors group (golang.org/x/sync/errgroup) for handling routines in main functions of Mainflux services.

2. Indicate the importance of this enhancement to you (must-have, should-have, nice-to-have).
   This is a nice-to-have.

Hi,

For safety use, it's better to have null check for all variables which are used in this function.

if (!_sessionPriv.$clientAlarmList) {
    return;
}

↓

if (!_sessionPriv.$clientAlarmList
     || !_sessionPriv.$monitoredItemForAlarmList
     || !_sessionPriv.$subscriptionforAlarmList ) {
    return;
}

https://github.com/node-opcua/node-opcua/blob/master/pack