Force Shutdown after accepting external changes

[lenhard]

JabRef 3.8.1-dev
windows 7 6.1 amd64
Java 1.8.0_92

External changes mean that JabRef will no longer shut down normally and you have to force kill the process.

Steps to reproduce:

1. Open a database in JabRef
2. Change the file externally
3. JabRef notices external changes
4. Review and accept all external changes
5. Try closing the window (click on X). JabRef refuses to shut down and the following message appears in the error console.

14:38:15.275 [AWT-EventQueue-0] INFO  net.sf.jabref.logic.importer.OpenDatabase - Opening: \\nas-a1\redirected$\jorglenh\Desktop\test.bib
14:38:39.993 [AWT-EventQueue-0] ERROR net.sf.jabref.FallbackExceptionHandler - Uncaught exception Occurred in Thread[AWT-EventQueue-0,6,main]
java.lang.IllegalArgumentException: missing event subscriber for an annotated method. Is net.sf.jabref.logic.autosaveandbackup.BackupManager@7772aed registered?
	at com.google.common.eventbus.SubscriberRegistry.unregister(SubscriberRegistry.java:111) ~[guava-20.0.jar:?]
	at com.google.common.eventbus.EventBus.unregister(EventBus.java:201) ~[guava-20.0.jar:?]
	at net.sf.jabref.model.metadata.MetaData.unregisterListener(MetaData.java:257) ~[main/:?]
	at net.sf.jabref.logic.autosaveandbackup.BackupManager.shutdown(BackupManager.java:149) ~[main/:?]
	at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184) ~[?:1.8.0_92]
	at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175) ~[?:1.8.0_92]
	at java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1548) ~[?:1.8.0_92]
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[?:1.8.0_92]
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[?:1.8.0_92]
	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151) ~[?:1.8.0_92]
	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174) ~[?:1.8.0_92]
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_92]
	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418) ~[?:1.8.0_92]
	at net.sf.jabref.logic.autosaveandbackup.BackupManager.shutdown(BackupManager.java:85) ~[main/:?]
	at net.sf.jabref.gui.JabRefFrame.quit(JabRefFrame.java:859) ~[main/:?]
	at net.sf.jabref.gui.JabRefFrame$CloseAction.actionPerformed(JabRefFrame.java:1778) ~[main/:?]
	at net.sf.jabref.gui.JabRefFrame$2.windowClosing(JabRefFrame.java:604) ~[main/:?]
	at java.awt.AWTEventMulticaster.windowClosing(AWTEventMulticaster.java:349) ~[?:1.8.0_92]
	at java.awt.Window.processWindowEvent(Window.java:2054) ~[?:1.8.0_92]
	at javax.swing.JFrame.processWindowEvent(JFrame.java:305) ~[?:1.8.0_92]
	at java.awt.Window.processEvent(Window.java:2013) ~[?:1.8.0_92]
	at java.awt.Component.dispatchEventImpl(Component.java:4889) ~[?:1.8.0_92]
	at java.awt.Container.dispatchEventImpl(Container.java:2294) ~[?:1.8.0_92]
	at java.awt.Window.dispatchEventImpl(Window.java:2746) ~[?:1.8.0_92]
	at java.awt.Component.dispatchEvent(Component.java:4711) ~[?:1.8.0_92]
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758) ~[?:1.8.0_92]
	at java.awt.EventQueue.access$500(EventQueue.java:97) ~[?:1.8.0_92]
	at java.awt.EventQueue$3.run(EventQueue.java:709) ~[?:1.8.0_92]
	at java.awt.EventQueue$3.run(EventQueue.java:703) ~[?:1.8.0_92]
	at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_92]
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76) ~[?:1.8.0_92]
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86) ~[?:1.8.0_92]
	at java.awt.EventQueue$4.run(EventQueue.java:731) ~[?:1.8.0_92]
	at java.awt.EventQueue$4.run(EventQueue.java:729) ~[?:1.8.0_92]
	at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_92]
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76) ~[?:1.8.0_92]
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:728) ~[?:1.8.0_92]
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201) [?:1.8.0_92]
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116) [?:1.8.0_92]
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105) [?:1.8.0_92]
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) [?:1.8.0_92]
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93) [?:1.8.0_92]
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82) [?:1.8.0_92]