add more logging for UrlManager to get more information for stack trace:
java.lang.NullPointerException
	at java.io.File.<init>(File.java:389)
	at org.freeplane.view.swing.features.filepreview.ChangeExternalImageAction.actionPerformed(ChangeExternalImageAction.java:54)
	at org.freeplane.core.ui.AccelerateableAction.actionPerformed(AccelerateableAction.java:177)
	[...]