Vaadin 7 Application doesn't work on apache tomcat 7 cluster

I am testing a small vaadin application in cluster environment and I use Apache Tomcat 7.0.42 server version, Vaadin 7.0.4 version,Java 1.6 and Apache as the load balancer. The problem is when I kill one tomcat instance, In webapp I can’t work continuously with In the current web page. I am getting a notification in browser telling that “Internal Problem take note of any unsaved data and click here to continue” and the below exception is printed in terminal. Allso I have to refresh my current page to recover. But the session data is replicated between the cluster and I am not re directed to the Login Page. Please Give a solution.

SEVERE:
javax.servlet.ServletException: No UIProvider returned a UI for the request.
at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:312)
at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:202)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:124)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.ha.session.JvmRouteBinderValve.invoke(JvmRouteBinderValve.java:220)
at org.apache.catalina.ha.tcp.ReplicationValve.invoke(ReplicationValve.java:335)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:954)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:120)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:919)
at java.lang.Thread.run(Thread.java:662)

Hi,
Does anyone has a solution for this problem because I am also facing the same problem…

Thanks