NPE when adding wms layer

[Cobra]

What steps will reproduce the problem?

1. add an wms layer

What is the expected result?
wms layer is added and displayed

What happens instead?
see below; layer isn't added while josm is still working

Please provide any additional information below. Attach a screenshot if
possible.
This happens only if you have used the layer with an older revision.
Deleting ~/.josm/wms-cache solves this problem.

Build-Date: 2011-06-29 10:59:35
Revision: 4183
Is-Local-Build: true

Identification: JOSM/1.5 (4183 SVN en)
Memory Usage: 124 MB / 910 MB (53 MB allocated, but free)
Java version: 1.6.0_24, Sun Microsystems Inc., Java HotSpot(TM) 64-Bit Server VM
Operating system: Linux
Dataset consistency test: No problems found

Plugin: ImproveWayAccuracy (26174)
Plugin: PicLayer (26174)
Plugin: alignways (26174)
Plugin: contourmerge (25626)
Plugin: measurement (26174)
Plugin: openstreetbugs (26174)
Plugin: openvisible (26174)
Plugin: proj4j (26174)
Plugin: reltoolbox (26174)
Plugin: reverter (26174)
Plugin: terracer (26174)
Plugin: undelete (26174)
Plugin: utilsplugin2 (26174)

java.lang.NullPointerException
	at org.openstreetmap.josm.data.imagery.WmsCache.removeNonReferencedFiles(WmsCache.java:238)
	at org.openstreetmap.josm.data.imagery.WmsCache.loadIndex(WmsCache.java:220)
	at org.openstreetmap.josm.gui.layer.WMSLayer.<init>(WMSLayer.java:135)
	at org.openstreetmap.josm.gui.layer.ImageryLayer.create(ImageryLayer.java:133)
	at org.openstreetmap.josm.actions.AddImageryLayerAction.actionPerformed(AddImageryLayerAction.java:27)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
	at javax.swing.AbstractButton.doClick(AbstractButton.java:357)
	at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:809)
	at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:850)
	at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
	at java.awt.Component.processMouseEvent(Component.java:6289)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6054)
	at java.awt.Container.processEvent(Container.java:2041)
	at java.awt.Component.dispatchEventImpl(Component.java:4652)
	at java.awt.Container.dispatchEventImpl(Container.java:2099)
	at java.awt.Component.dispatchEvent(Component.java:4482)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
	at java.awt.Container.dispatchEventImpl(Container.java:2085)
	at java.awt.Window.dispatchEventImpl(Window.java:2478)
	at java.awt.Component.dispatchEvent(Component.java:4482)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:644)
	at java.awt.EventQueue.access$000(EventQueue.java:85)
	at java.awt.EventQueue$1.run(EventQueue.java:603)
	at java.awt.EventQueue$1.run(EventQueue.java:601)
	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$2.run(EventQueue.java:617)
	at java.awt.EventQueue$2.run(EventQueue.java:615)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:614)
	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

[jttt]

In [4186/josm]:

Fix #6522 NPE when adding wms layer