Fixing another apparently harmless exception printed from acquireWorkspace.
INFO: demo #3 completed: SUCCESS
Exception in thread "CPS VM execution thread: org.jenkinsci.plugins.workflow.cps.CpsFlowExecution@1c855b9a" java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask@12b56d39 rejected from java.util.concurrent.ScheduledThreadPoolExecutor@4515c2c7[Shutting down, pool size = 7, active threads = 0, queued tasks = 0, completed tasks = 51]
	at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2048)
	at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:821)
	at java.util.concurrent.ScheduledThreadPoolExecutor.delayedExecute(ScheduledThreadPoolExecutor.java:325)
	at java.util.concurrent.ScheduledThreadPoolExecutor.schedule(ScheduledThreadPoolExecutor.java:530)
	at java.util.concurrent.ScheduledThreadPoolExecutor.submit(ScheduledThreadPoolExecutor.java:629)
	at hudson.remoting.SingleLaneExecutorService.execute(SingleLaneExecutorService.java:104)
	at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:110)
	at org.jenkinsci.plugins.workflow.flow.FlowExecutionList.saveLater(FlowExecutionList.java:104)
	at org.jenkinsci.plugins.workflow.flow.FlowExecutionList.unregister(FlowExecutionList.java:100)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.finish(WorkflowRun.java:386)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.access$800(WorkflowRun.java:95)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun$GraphL.onNewHead(WorkflowRun.java:613)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.notifyListeners(CpsFlowExecution.java:594)
	at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$4.run(CpsThreadGroup.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:745)