Diagnosis for random NPE in test.
https://jenkins.ci.cloudbees.com/job/plugins/job/workflow-plugin/org.jenkins-ci.plugins.workflow$workflow-cps/721/testReport/junit/org.jenkinsci.plugins.workflow.cps/CpsScriptTest/evaluateShallBeCpsTransformed/
java.lang.NullPointerException: Cannot invoke method get() on null object
	at org.codehaus.groovy.runtime.NullObject.invokeMethod(NullObject.java:77)
	at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:45)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
	at org.codehaus.groovy.runtime.callsite.NullCallSite.call(NullCallSite.java:32)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:108)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:112)
	at org.jenkinsci.plugins.workflow.cps.CpsScriptTest.evaluateShallBeCpsTransformed(CpsScriptTest.groovy:71)