Not sure how it can happen, but the following is a stacktrace from our logs... look like this happened a few times in the past:

ERROR net.sourceforge.stripes.exception.DefaultExceptionHandler ?(?:?) 27.02.2008 07:44:01 java.lang.NullPointerException: null
java.lang.NullPointerException
at java.lang.String.startsWith(String.java:1422)
at java.lang.String.startsWith(String.java:1451)
at info.magnolia.cms.beans.config.URI2RepositoryMapping.matches(URI2RepositoryMapping.java:86)
at info.magnolia.cms.beans.config.URI2RepositoryManager.getMapping(URI2RepositoryManager.java:83)
at info.magnolia.cms.filters.RepositoryMappingFilter.doFilter(RepositoryMappingFilter.java:77)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:70)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)
at info.magnolia.cms.filters.CompositeFilter.doFilter(CompositeFilter.java:64)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:70)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:65)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.filters.CompositeFilter.doFilter(CompositeFilter.java:64)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:70)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)
at info.magnolia.cms.filters.VirtualUriFilter.doFilter(VirtualUriFilter.java:117)
at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:62)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)
at info.magnolia.cms.cache.CacheFilter.doFilter(CacheFilter.java:134)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:70)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at info.magnolia.cms.i18n.I18nContentSupportFilter.doFilter(I18nContentSupportFilter.java:69)
at info.magnolia.cms.filters.AbstractMgnlFilter.doFilter(AbstractMgnlFilter.java:70)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:73)
at net.sourceforge.stripes.controller.StripesFilter.doFilter(StripesFilter.java:180)
at info.magnolia.cms.filters.MgnlFilterChain.doFilter(MgnlFilterChain.java:71)

A null check in URI2RepositoryMapping.matches will not hurt...