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.

javax.servlet.ServletException: No UIProvider returned a UI for the request.
at com.vaadin.server.VaadinServlet.service(
at com.vaadin.server.VaadinServlet.service(
at javax.servlet.http.HttpServlet.service(
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(
at org.apache.catalina.core.ApplicationFilterChain.doFilter(
at org.apache.catalina.core.StandardWrapperValve.invoke(
at org.apache.catalina.core.StandardContextValve.invoke(
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(
at org.apache.catalina.core.StandardHostValve.invoke(
at org.apache.catalina.ha.session.JvmRouteBinderValve.invoke(
at org.apache.catalina.ha.tcp.ReplicationValve.invoke(
at org.apache.catalina.valves.ErrorReportValve.invoke(
at org.apache.catalina.valves.AccessLogValve.invoke(
at org.apache.catalina.core.StandardEngineValve.invoke(
at org.apache.catalina.connector.CoyoteAdapter.service(
at org.apache.coyote.http11.AbstractHttp11Processor.process(
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(
at java.util.concurrent.ThreadPoolExecutor$

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