Better diagnosis for null parent.
Observed in 1.509.3:
java.lang.NullPointerException
	at hudson.model.AbstractItem.getRootDir(AbstractItem.java:162)
	at hudson.model.Items.getConfigFile(Items.java:236)
	at hudson.model.AbstractItem.getConfigFile(AbstractItem.java:461)
	at hudson.model.AbstractItem.save(AbstractItem.java:456)
	at hudson.model.Job.save(Job.java:152)
	at hudson.model.AbstractProject.save(AbstractProject.java:268)
	at hudson.diagnosis.OldDataMonitor.doDiscard(OldDataMonitor.java:275)
Merge branch 'master' of github.com:jenkinsci/jenkins
Missing @since.
Merge branch 'master' of github.com:jenkinsci/jenkins
Deprecating hudson field; use jenkins instead.
It is Thread.getThreadGroup that can return null, not ThreadGroup.getName.
https://jenkins.ci.cloudbees.com/job/core/job/jenkins_main_trunk/1436/testReport/hudson.model/UpdateCenter2Test/install/
java.lang.NullPointerException: null
	at hudson.Functions$ThreadSorterBase.<init>(Functions.java:1151)
	at hudson.Functions$ThreadSorterBase.<init>(Functions.java:1142)
	at hudson.Functions$ThreadGroupMap.<init>(Functions.java:1163)
	at hudson.Functions.sortThreadsAndGetGroupMap(Functions.java:1136)
	at org.jvnet.hudson.test.JenkinsRule$2.evaluate(JenkinsRule.java:482)