NEutron VPN / upon subnetupdate, sometimes networkID is not set

https://git.opendaylight.org/gerrit/#/c/62949/

I could not find the traces that made me open this issue.

however, this was an exception that occured on subnetmapchangelistener ( from vpnmanager).
when the code attempts to access to getNetworkId().getValue(), the problem arose.

protected void update(InstanceIdentifier<Subnetmap> identifier, Subnetmap subnetmapOriginal, Subnetmap
subnetmapUpdate) {
LOG.trace("update:SubnetmapListener update subnetmap method - key: {}, original: {}, update: {}",
identifier, subnetmapOriginal, subnetmapUpdate);
...
String elanInstanceName = subnetmapUpdate.getNetworkId().getValue();