FD_SOCK NullPointerException during shutdown

"FD_SOCK acceptor-11,LeaveTest,1@5864" daemon prio=5 tid=0x885 nid=NA runnable
  java.lang.Thread.State: RUNNABLE
	  at java.lang.NullPointerException.<init>(NullPointerException.java:60)
	  at org.jgroups.protocols.FD_SOCK$ServerSocketHandler.run(FD_SOCK.java:996)
	  at java.lang.Thread.run(Thread.java:748)

"main@1" prio=5 tid=0x1 nid=NA waiting
  java.lang.Thread.State: WAITING
	  at java.lang.Object.wait(Object.java:-1)
	  at java.lang.Thread.join(Thread.java:1260)
	  at org.jgroups.protocols.FD_SOCK.stopPingerThread(FD_SOCK.java:544)
	  - locked <0x1858> (a org.jgroups.protocols.FD_SOCK)
	  at org.jgroups.protocols.FD_SOCK.stop(FD_SOCK.java:218)
	  at org.jgroups.stack.ProtocolStack.removeProtocol(ProtocolStack.java:692)
	  at org.jgroups.stack.ProtocolStack.removeProtocol(ProtocolStack.java:678)
	  at org.jgroups.stack.ProtocolStack.removeProtocols(ProtocolStack.java:659)
	  at org.jgroups.tests.LeaveTest.lambda$testLeaveOfSecondHalfWithCoordLeaving$6(LeaveTest.java:134)