I haven't really had time to look into this but here's how I got into this
situation:
. downloaded and deployed hudson 1.129
. created a matrix project
. filled in the title and svn repository information
. saved and then:

Aug 16, 2007 3:24:57 PM org.apache.catalina.core.StandardContext listenerStart
SEVERE: Exception sending context initialized event to listener instance of
class hudson.WebAppMain
java.lang.NullPointerException
at hudson.matrix.MatrixProject.getRootDirFor(MatrixProject.java:262)
at hudson.matrix.MatrixProject.getRootDirFor(MatrixProject.java:257)
at hudson.matrix.MatrixProject.getRootDirFor(MatrixProject.java:55)
at hudson.model.AbstractItem.getRootDir(AbstractItem.java:56)
at hudson.model.AbstractItem.doSetName(AbstractItem.java:85)
at hudson.model.AbstractItem.<init>(AbstractItem.java:42)
at hudson.model.Job.<init>(Job.java:92)
at hudson.model.AbstractProject.<init>(AbstractProject.java:131)
at hudson.model.Project.<init>(Project.java:48)
at hudson.matrix.MatrixConfiguration.<init>(MatrixConfiguration.java:35)
at hudson.matrix.MatrixProject.rebuildConfigurations(MatrixProject.java:217)
at hudson.matrix.MatrixProject.onLoad(MatrixProject.java:120)
at hudson.model.Items.load(Items.java:80)
at hudson.model.Hudson.load(Hudson.java:1010)
at hudson.model.Hudson.<init>(Hudson.java:230)
at hudson.WebAppMain.contextInitialized(WebAppMain.java:105)
at
org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3827)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4336)
at
org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:760)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:740)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:525)
at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:825)
at org.apache.catalina.startup.HostConfig.deployWARs(HostConfig.java:714)
at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:490)
at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1138)
at
org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:311)
at
org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:120)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1022)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:719)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1014)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:443)
at org.apache.catalina.core.StandardService.start(StandardService.java:448)
at org.apache.catalina.core.StandardServer.start(StandardServer.java:710)
at org.apache.catalina.startup.Catalina.start(Catalina.java:552)
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:585)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)

the funny thing is that it prevents restarting hudson, rendering the whole thing
unusable.