NullPointerException in HostTracker.java

Two buggy sites may raise NullPointerException in HostTracker module if concurrent remove operation occurs.

The first case is for "transHost" variable, if a concurrent remove upon "hostsDB" variable. The corresponding snippets are attached:

HostNodeConnector transHost = hostsDB.get(networkAddr);
transHost.setStaticHost(true);

The second case is for "existinghost" variable. The following are snippets:

HostNodeConnector existinghost = hostsDB.get(id);
existinghost.initArpSendCountDown();

To solve the issue, the simplest way is to add a null checker for these variable since we cannot guarantee the result from hostsDB is not NULL.