error ejecutando Struts2

[asego]

Buenas tardes,

Soy nuevo en este tema del struts2, estoy leyendo un manual, pero me da el siguiente error:

GRAVE: Excepción arrancando filtro struts2
java.lang.NullPointerException
at com.opensymphony.xwork2.util.FileManager$FileRevis ion.needsReloading(FileManager.java:209)
at com.opensymphony.xwork2.util.FileManager.fileNeeds Reloading(FileManager.java:60)
at com.opensymphony.xwork2.config.providers.XmlConfig urationProvider.needsReload(XmlConfigurationProvid er.java:325)
at org.apache.struts2.config.StrutsXmlConfigurationPr ovider.needsReload(StrutsXmlConfigurationProvider. java:169)
at com.opensymphony.xwork2.config.ConfigurationManage r.conditionalReload(ConfigurationManager.java:220)
at com.opensymphony.xwork2.config.ConfigurationManage r.getConfiguration(ConfigurationManager.java:61)
at org.apache.struts2.dispatcher.Dispatcher.getContai ner(Dispatcher.java:901)
at org.apache.struts2.dispatcher.ng.InitOperations.in itStaticContentLoader(InitOperations.java:82)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrep areAndExecuteFilter.init(StrutsPrepareAndExecuteFi lter.java:52)
at org.apache.catalina.core.ApplicationFilterConfig.g etFilter(ApplicationFilterConfig.java:275)
at org.apache.catalina.core.ApplicationFilterConfig.s etFilterDef(ApplicationFilterConfig.java:397)
at org.apache.catalina.core.ApplicationFilterConfig.< init>(ApplicationFilterConfig.java:108)
at org.apache.catalina.core.StandardContext.filterSta rt(StandardContext.java:3800)
at org.apache.catalina.core.StandardContext.start(Sta ndardContext.java:4450)
at org.apache.catalina.core.ContainerBase.start(Conta inerBase.java:1045)
at org.apache.catalina.core.StandardHost.start(Standa rdHost.java:722)
at org.apache.catalina.core.ContainerBase.start(Conta inerBase.java:1045)
at org.apache.catalina.core.StandardEngine.start(Stan dardEngine.java:443)
at org.apache.catalina.core.StandardService.start(Sta ndardService.java:516)
at org.apache.catalina.core.StandardServer.start(Stan dardServer.java:710)
at org.apache.catalina.startup.Catalina.start(Catalin a.java:583)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.catalina.startup.Bootstrap.start(Bootst rap.java:288)
at org.apache.catalina.startup.Bootstrap.main(Bootstr ap.java:413)

He estado buscando la solucion, y no se porque se produce este error. ¿Alguien sabe como solucionarlo?

Muchas gracias. Un saludo.