Correction of ConcurrentModificationException in Context collection traversal

Correction of the following exception:
java.util.ConcurrentModificationException
	at
java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1115)
	at java.util.TreeMap$ValueIterator.next(TreeMap.java:1160)
	at
com.google.common.collect.Iterators$7.computeNext(Iterators.java:646)
	at
com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
	at
com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
	at
io.janusproject.kernel.JanusSpawnService.fireAgentDestroyed(JanusSpawnService.java:216)
	at
io.janusproject.kernel.JanusSpawnService.killAgent(JanusSpawnService.java:102)
	at
io.janusproject.kernel.bic.LifecycleSkill.killMe(LifecycleSkill.java:64)
	at
io.sarl.demos.filesearch.FileSearchAgent.killMe(FileSearchAgent.java:108)
	at
io.sarl.demos.filesearch.FileSearchAgent._handle_Stop_4(FileSearchAgent.java:415)
	at sun.reflect.GeneratedMethodAccessor13.invoke(Unknown Source)
	at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at
com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
	at
com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
	at
com.google.common.eventbus.EventBus.dispatch(EventBus.java:332)
	at
com.google.common.eventbus.AsyncEventBus.access$001(AsyncEventBus.java:34)
	at
com.google.common.eventbus.AsyncEventBus$1.run(AsyncEventBus.java:118)
	at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:744)