NPE when closing JOSM with history dialog still opened

[Don-vip]

What steps will reproduce the problem?

1. Have reverter plugin installed
2. Revert changeset 15875597 (see #8676)
3. Undo
4. Show history dialog of node 885644779 (tested on second monitor, with also command stack opened on this monitor)
5. Redo
6. Exit JOSM

--> NPE

Build-Date: 2013-05-06 00:01:52
Revision: 5927
Is-Local-Build: true

Identification: JOSM/1.5 (5927 SVN en_GB) Windows 8 64-Bit
Memory Usage: 266 MB / 1813 MB (110 MB allocated, but free)
Java version: 1.6.0_45, Sun Microsystems Inc., Java HotSpot(TM) 64-Bit Server VM
VM arguments: [-Dfile.encoding=UTF-8]

Plugin: DirectDownload (29435)
Plugin: DirectUpload (29435)
Plugin: FixAddresses (29519)
Plugin: HouseNumberTaggingTool (29487)
Plugin: OpeningHoursEditor (29435)
Plugin: RoadSigns (29435)
Plugin: TombPlugin (41)
Plugin: Tracer (29435)
Plugin: alignways (29435)
Plugin: buildings_tools (29435)
Plugin: cadastre-fr (29526)
Plugin: contourmerge (1003)
Plugin: download_along (29435)
Plugin: ext_tools (29435)
Plugin: geotools (29435)
Plugin: gpxfilter (29435)
Plugin: imagery-xml-bounds (29461)
Plugin: jts (29435)
Plugin: log4j (29435)
Plugin: measurement (29545)
Plugin: merge-overlap (29435)
Plugin: michigan_left (29435)
Plugin: mirrored_download (29505)
Plugin: multipoly-convert (29435)
Plugin: opendata (29505)
Plugin: openstreetbugs (29435)
Plugin: openvisible (29435)
Plugin: print (29529)
Plugin: proj4j (29435)
Plugin: public_transport (29435)
Plugin: reltoolbox (29535)
Plugin: reverter (29553)
Plugin: tag2link (29461)
Plugin: terracer (29509)
Plugin: turnlanes (29435)
Plugin: turnrestrictions (29435)
Plugin: undelete (29555)
Plugin: utilsplugin2 (29435)
Plugin: videomapping (29536)
Plugin: waydownloader (29435)
Plugin: waypoint_search (29435)
Plugin: wayselector (29435)
Plugin: wikipedia (29523)

java.lang.NullPointerException
	at java.util.Calendar.setTime(Calendar.java:1076)
	at java.text.SimpleDateFormat.format(SimpleDateFormat.java:875)
	at java.text.SimpleDateFormat.format(SimpleDateFormat.java:868)
	at java.text.DateFormat.format(DateFormat.java:316)
	at org.openstreetmap.josm.gui.history.VersionInfoPanel.getInfoText(VersionInfoPanel.java:99)
	at org.openstreetmap.josm.gui.history.VersionInfoPanel.update(VersionInfoPanel.java:131)
	at java.util.Observable.notifyObservers(Observable.java:142)
	at java.util.Observable.notifyObservers(Observable.java:98)
	at org.openstreetmap.josm.gui.history.HistoryBrowserModel.fireModelChange(HistoryBrowserModel.java:212)
	at org.openstreetmap.josm.gui.history.HistoryBrowserModel.activeLayerChange(HistoryBrowserModel.java:754)
	at org.openstreetmap.josm.gui.MapView.fireActiveLayerChanged(MapView.java:164)
	at org.openstreetmap.josm.gui.MapView.setActiveLayer(MapView.java:776)
	at org.openstreetmap.josm.gui.MapView.removeLayer(MapView.java:405)
	at org.openstreetmap.josm.Main.removeLayer(Main.java:296)
	at org.openstreetmap.josm.Main.exitJosm(Main.java:755)
	at org.openstreetmap.josm.gui.MainApplication$1.windowClosing(MainApplication.java:86)
	at java.awt.AWTEventMulticaster.windowClosing(AWTEventMulticaster.java:332)
	at java.awt.Window.processWindowEvent(Window.java:1871)
	at javax.swing.JFrame.processWindowEvent(JFrame.java:274)
	at java.awt.Window.processEvent(Window.java:1829)
	at java.awt.Component.dispatchEventImpl(Component.java:4660)
	at java.awt.Container.dispatchEventImpl(Container.java:2097)
	at java.awt.Window.dispatchEventImpl(Window.java:2489)
	at java.awt.Component.dispatchEvent(Component.java:4488)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:674)
	at java.awt.EventQueue.access$400(EventQueue.java:81)
	at java.awt.EventQueue$2.run(EventQueue.java:633)
	at java.awt.EventQueue$2.run(EventQueue.java:631)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
	at java.awt.EventQueue$3.run(EventQueue.java:647)
	at java.awt.EventQueue$3.run(EventQueue.java:645)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:644)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

[Don-vip]

In 5929/josm:

fix #8677 - NPE in VersionHistoryPanel when undoing/redoing changeset revert