美国山羊人:struts tiles plug-in 出错

在我的struts-config.xml 中加入了tiles plug-in，文件内容如下：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts-config PUBLIC "-//Apache Software Foundation//DTD Struts Configuration 1.1//EN" "http://jakarta.apache.org/struts/dtds/struts-config_1_1.dtd">
<struts-config>

<form-beans>
<form-bean name="LoginForm" type="asset.form.LoginForm" />
</form-beans>

<global-forwards>
<forward name="index" path="/index.jsp" redirect="true" />
<forward name="login" path="/login.jsp" redirect="true" />
<forward name="logout" path="/logout.do" redirect="true" />
<forward name="mainframe" path="/mainframe.jsp" redirect="true" />
<forward name="main" path="/main.jsp" redirect="true" />
<forward name="register" path="/register.jsp" redirect="true" />
</global-forwards>

<action-mappings>
<action path="/index" forward="/index.jsp" />

<action path="/authenticate"
type="asset.action.UserAction"
name="LoginForm"
scope="request"
validate="true"
input="/login.jsp">
</action>
</action-mappings>

<plug-in className="org.apache.struts.tiles.TilesPlugin">
<set-property property="definitions-config" value="/WEB-INF/tiles-defs.xml />
<set-property property="moduleAware" value="true" />
<set-property property="definitions-parser-validate" value="true" />
</plug-in>

</struts-config>

不但tiles plug-in 没起作用，连原有的ActionMappings 和ActionFormBeans都提示找不到了。谁知道是什么原因吗？
页面出错：
java.lang.NullPointerException
org.apache.struts.taglib.tiles.InsertTag.processAttribute(InsertTag.java:689)
org.apache.struts.taglib.tiles.InsertTag.createTagHandler(InsertTag.java:489)
org.apache.struts.taglib.tiles.InsertTag.doStartTag(InsertTag.java:451)
org.apache.jsp.classicLayout_jsp._jspx_meth_tiles_insert_0(org.apache.jsp.classicLayout_jsp:118)
org.apache.jsp.classicLayout_jsp._jspService(org.apache.jsp.classicLayout_jsp:71)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:322)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)