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)