Trace Model Concurrent Modification Exception

[dingyuchen]

Met with ConcurrentModificationException when running microbat, console output is as such:

java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911)
	at java.util.ArrayList$Itr.next(ArrayList.java:861)
	at microbat.model.trace.Trace.constructControlDomianceRelation(Trace.java:173)
  	at microbat.instrumentation.TraceAgent.constructTrace(TraceAgent.java:92)
	at microbat.instrumentation.TraceAgent.shutdown(TraceAgent.java:69)
 	at microbat.instrumentation.Agent.stop(Agent.java:77)
	at microbat.instrumentation.Agent._exitProgram(Agent.java:56)
	at exp.concurrency.case1.DiningPhilosophersDebug.main(DiningPhilosophersDebug.java:18)

ArrayList used may require synchronization