[JSP]getElementById è null

[valeof]

Ciao a tutti.

Ho una pagina jsp(che chiameremo A) che apre una popup(che chiameremo B), la quale passa per una servlet.
La serlvet reindirizza a una jsp(che chiameremo C) mantenendo aperta A

Nella jsp chiamante(A) ho:

  <TR>
	  <TD colspan="10">
		<FONT color="red"><INPUT id="msgErrorCoperturaPolizza" style="border: none; color: red; font-weight: bold; background-color:#bed3df;" readonly="readonly" value="" size="100" ></FONT>
	  </TD>
  </TR>

Nella jsp C ho:

<%
String errore = (String)session.getAttribute("erroreCoperturaPolizza");
		if(errore!=null)
		{
		%>
			alert('messaggio errore: '+top.opener.document.getElementById('msgErrorCoperturaPolizza').value);
			try{
				top.opener.document.getElementById('msgErrorCoperturaPolizza').value = '<%= errore %>';
			}catch(Exception){}
		
		<% } %>

L'alert non funziona cosi ma funziona se scrivo

			alert('messaggio errore: '+top.opener.document.getElementById('msgErrorCoperturaPolizza'));

e mi da "object".
Inoltre se faccio CNTRL+N sulla jsp C mi dà NullPointerException:

java.lang.NullPointerException
	at java.lang.Throwable.<init>(Throwable.java)
	at java.lang.Throwable.<init>(Throwable.java)
	at java.lang.NullPointerException.<init>(NullPointerException.java:60)
	at org.apache.jsp._SisPragmaDett._jspService(SisPragmaDett.jsp  :78)
	at com.ibm.ws.webcontainer.jsp.runtime.HttpJspBase.service(HttpJspBase.java:89)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
	at com.ibm.ws.webcontainer.jsp.servlet.JspServlet$JspServletWrapper.service(JspServlet.java:344)
	at com.ibm.ws.webcontainer.jsp.servlet.JspServlet.serviceJspFile(JspServlet.java:669)
	at com.ibm.ws.webcontainer.jsp.servlet.JspServlet.service(JspServlet.java:767)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
	at com.ibm.ws.webcontainer.servlet.StrictServletInstance.doService(StrictServletInstance.java:110)
	at com.ibm.ws.webcontainer.servlet.StrictLifecycleServlet._service(StrictLifecycleServlet.java:174)
	at com.ibm.ws.webcontainer.servlet.IdleServletState.service(StrictLifecycleServlet.java:313)
	at com.ibm.ws.webcontainer.servlet.StrictLifecycleServlet.service(StrictLifecycleServlet.java:116)
	at com.ibm.ws.webcontainer.servlet.ServletInstance.service(ServletInstance.java:283)
	at com.ibm.ws.webcontainer.servlet.ValidServletReferenceState.dispatch(ValidServletReferenceState.java:42)
	at com.ibm.ws.webcontainer.servlet.ServletInstanceReference.dispatch(ServletInstanceReference.java:40)
	at com.ibm.ws.webcontainer.webapp.WebAppRequestDispatcher.handleWebAppDispatch(WebAppRequestDispatcher.java)
	at com.ibm.ws.webcontainer.webapp.WebAppRequestDispatcher.dispatch(WebAppRequestDispatcher.java)
	at com.ibm.ws.webcontainer.webapp.WebAppRequestDispatcher.forward(WebAppRequestDispatcher.java)
	at com.ibm.ws.webcontainer.srt.WebAppInvoker.doForward(WebAppInvoker.java:119)
	at com.ibm.ws.webcontainer.srt.WebAppInvoker.handleInvocationHook(WebAppInvoker.java:276)
	at com.ibm.ws.webcontainer.cache.invocation.CachedInvocation.handleInvocation(CachedInvocation.java:71)
	at com.ibm.ws.webcontainer.cache.invocation.CacheableInvocationContext.invoke(CacheableInvocationContext.java:116)
	at com.ibm.ws.webcontainer.srp.ServletRequestProcessor.dispatchByURI(ServletRequestProcessor.java:186)
	at com.ibm.ws.webcontainer.oselistener.OSEListenerDispatcher.service(OSEListener.java:334)
	at com.ibm.ws.webcontainer.http.HttpConnection.handleRequest(HttpConnection.java:56)
	at com.ibm.ws.http.HttpConnection.readAndHandleRequest(HttpConnection.java:618)
	at com.ibm.ws.http.HttpConnection.run(HttpConnection.java)
	at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:672)

Ciao a tutti e grazie anticipatamente