Jenkins fails to restart when libvirt-slave-plugin 1.8.5 is installed. Logfile contains the following lines:

Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins cleanUp
INFO: Stopping Jenkins
Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$21 onAttained
INFO: Started termination
Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$21 onAttained
INFO: Completed termination
Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins _cleanUpDisconnectComputers
INFO: Starting node disconnection
Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$22 run
WARNING: Could not disconnect hudson.plugins.libvirt.VirtualMachineSlaveComputer@17dc390b: null
java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
at hudson.lifecycle.UnixLifecycle.restart(UnixLifecycle.java:70)
at jenkins.model.Jenkins$26.run(Jenkins.java:4145)

Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$22 run
WARNING: Could not disconnect hudson.plugins.libvirt.VirtualMachineSlaveComputer@6344b0b7: null
java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
at hudson.lifecycle.UnixLifecycle.restart(UnixLifecycle.java:70)
at jenkins.model.Jenkins$26.run(Jenkins.java:4145)

Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$22 run
WARNING: Could not disconnect hudson.plugins.libvirt.VirtualMachineSlaveComputer@39bc8ceb: null
java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
at hudson.lifecycle.UnixLifecycle.restart(UnixLifecycle.java:70)
at jenkins.model.Jenkins$26.run(Jenkins.java:4145)

Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$22 run
WARNING: Could not disconnect hudson.plugins.libvirt.VirtualMachineSlaveComputer@4a2c3c84: null
java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
at hudson.lifecycle.UnixLifecycle.restart(UnixLifecycle.java:70)
at jenkins.model.Jenkins$26.run(Jenkins.java:4145)
Dec 26, 2016 4:04:44 AM jenkins.model.Jenkins$26 run
WARNING: Failed to restart Jenkins
java.lang.RuntimeException: Unexpected issues encountered during cleanUp: null; null; null; null
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3202)
at hudson.lifecycle.UnixLifecycle.restart(UnixLifecycle.java:70)
at jenkins.model.Jenkins$26.run(Jenkins.java:4145)
Suppressed: java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
... 2 more
Suppressed: java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
... 2 more
Suppressed: java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
... 2 more
Caused by: java.lang.NullPointerException
at hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect(VirtualMachineSlaveComputer.java:59)
at jenkins.model.Jenkins$22.run(Jenkins.java:3296)
at hudson.model.Queue._withLock(Queue.java:1303)
at hudson.model.Queue.withLock(Queue.java:1180)
at jenkins.model.Jenkins._cleanUpDisconnectComputers(Jenkins.java:3289)
at jenkins.model.Jenkins.cleanUp(Jenkins.java:3165)
... 2 more

Root cause:

jenkins.model.Jenkins object enumerates over all hudson.model.Computer instances and calls hudson.model.Computer.disconnect() method with argument cause=null (line 3296). But method hudson.plugins.libvirt.VirtualMachineSlaveComputer.disconnect() doesn't expect cause argument to be null (line 59). As a result, Java raises java.lang.NullPointerException because of null pointer dereference.

jenkins.model.Jenkins:run() method near line 3296:

                for( Computer c : computers.values() ) {
                    try {
                        c.interrupt();
                        killComputer(c);
                        pending.add(c.disconnect(null)); // <----------line 3296: call disconnect() with cause=null
                    } catch (OutOfMemoryError e) {

hudson.plugins.libvirt.VirtualMachineSlaveComputer:disconnect() method near line 59:

	@Override
	public Future<?> disconnect(OfflineCause cause) {
		VirtualMachineSlave slave = (VirtualMachineSlave) getNode();
		String virtualMachineName = slave.getVirtualMachineName();
		VirtualMachineLauncher vmL = (VirtualMachineLauncher) getLauncher();
		Hypervisor hypervisor = vmL.findOurHypervisorInstance();
		logger.log(Level.INFO, "Virtual machine \"" + virtualMachineName + "\" (slave \"" + getDisplayName() + "\") is to be shut down. reason: "+cause+" ("+cause.getClass().getName()+")"); // <-------- line 59: cause.getClass() raises exception, as cause=null
		taskListener.getLogger().println("Virtual machine \"" + virtualMachineName + "\" (slave \"" + getDisplayName() + "\") is to be shut down.");