Java.lang.NullPointerException:Module ' null ' not found.

By the module ' null ' not found the meaning of <load-on-startup> is understood

After using the HTML tag, always in the 
 
Org.apache.jasper.JasperException:java.lang.NullPointerException:Module 'null' Not found.
	Org.apache.jasper.servlet.JspServletWrapper.handleJspException (jspservletwrapper.java:522)
	Org.apache.jasper.servlet.JspServletWrapper.service (jspservletwrapper.java:416)
	Org.apache.jasper.servlet.JspServlet.serviceJspFile (jspservlet.java:337)
	Org.apache.jasper.servlet.JspServlet.service (jspservlet.java:266)
	Javax.servlet.http.HttpServlet.service ( httpservlet.java:803)
 
 
 java.lang.NullPointerException:Module 'null' not found.
	Org.apache.struts.taglib.TagUtils.getModuleConfig (tagutils.java:755)
	Org.apache.struts.taglib.TagUtils.getModuleConfig (tagutils.java:735) Org.apache.struts.taglib.html.FormTag.lookup (formtag.java:796) Org.apache.struts.taglib.html.FormTag.doStartTag (formtag.java:467) org.apache.jsp.jsp_jsp._jspx_meth_html_005fform_005f0 (jsp_jsp.java:117) org.apache.jsp.jsp_
	Jsp._jspservice (jsp_jsp.java:86) org.apache.jasper.runtime.HttpJspBase.service (httpjspbase.java:70)
	Javax.servlet.http.HttpServlet.service (httpservlet.java:803)
	Org.apache.jasper.servlet.JspServletWrapper.service (jspservletwrapper.java:374)
	Org.apache.jasper.servlet.JspServlet.serviceJspFile (jspservlet.java:337) Org.apache.jasper.servlet.JspServlet.service (jspservlet.java:266) Javax.servlet.http.HttpServlet.service ( httpservlet.java:803) 
 
 solve: Less <load-on-startup>0</load-on-startup> Why add this sentence, sometimes not add also can be executed. Because in the servlet configuration, <load-on-startup>0</load-on-startup>, the tag container loads the servlet at 0 or greater than 0 o'clock when it is started. Indicates that the container loads the servlet when the application starts when it is a negative number, or when unspecified, indicates that the container is loaded when the servlet is selected, and the smaller the value of a positive number, the higher the priority of starting the servlet. If it is not written, it is loaded when the servlet is invoked, and the struts tag is used on the page, and the servlet is not loaded so it complains.