when watching a page, Xwiki sends a velocity error instead of the actual change

We recently upgraded to 1.8.1 and one of my users reported this bug and I can confirm it. When watching a page, Xwiki send out a notification about the change with the following subject and body:

Subject: XWiki : Watchlist Updates, 16/04/2009 05:00

Error number 4001 in 4: Error while parsing velocity page XWiki.WatchListMessage Wrapped Exception: Failed to evaluate content with id [XWiki.WatchListMessage]

Error number 4001 in 4: Error while parsing velocity page XWiki.WatchListMessage
Wrapped Exception: Failed to evaluate content with id [XWiki.WatchListMessage]
com.xpn.xwiki.XWikiException: Error number 4001 in 4: Error while parsing velocity page XWiki.WatchListMessage
Wrapped Exception: Failed to evaluate content with id [XWiki.WatchListMessage]
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:114)
at com.xpn.xwiki.plugin.mailsender.MailSenderPlugin.sendMailFromTemplate(MailSenderPlugin.java:663)
at com.xpn.xwiki.plugin.watchlist.WatchListJob.sendNotificationMessage(WatchListJob.java:430)
at com.xpn.xwiki.plugin.watchlist.WatchListJob.execute(WatchListJob.java:181)
at org.quartz.core.JobRunShell.run(JobRunShell.java:202)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:529)

Wrapped Exception:

org.apache.velocity.exception.MethodInvocationException: Invocation of method 'getExternalURL' in class com.xpn.xwiki.api.Document threw exception java.lang.NullPointerException @ XWiki.WatchListMessage[33,138]
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:286)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74)
at org.apache.velocity.runtime.directive.Foreach.render(Foreach.java:448)
at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.xwiki.velocity.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:178)
at org.xwiki.velocity.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:143)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:108)
at com.xpn.xwiki.plugin.mailsender.MailSenderPlugin.sendMailFromTemplate(MailSenderPlugin.java:663)
at com.xpn.xwiki.plugin.watchlist.WatchListJob.sendNotificationMessage(WatchListJob.java:430)
at com.xpn.xwiki.plugin.watchlist.WatchListJob.execute(WatchListJob.java:181)
at org.quartz.core.JobRunShell.run(JobRunShell.java:202)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:529)
Caused by: java.lang.NullPointerException
at com.xpn.xwiki.XWiki.getServletPath(XWiki.java:4368)
at com.xpn.xwiki.web.XWikiServletURLFactory.addServletPath(XWikiServletURLFactory.java:208)
at com.xpn.xwiki.web.XWikiServletURLFactory.createURL(XWikiServletURLFactory.java:178)
at com.xpn.xwiki.web.XWikiServletURLFactory.createExternalURL(XWikiServletURLFactory.java:273)
at com.xpn.xwiki.doc.XWikiDocument.getExternalURL(XWikiDocument.java:992)
at com.xpn.xwiki.api.Document.getExternalURL(Document.java:591)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:245)
... 14 more