Description of problem:
I got ISE 500 when i forget choose system on page  Configuration - Configuration Files - Compare File - Deployed Files

Version-Release number of selected component (if applicable):
spacewalk-java-2.3.8-100.el6sat.noarch

How reproducible:
always

Steps to Reproduce:
1. go to Configuration ->  Configuration Files -> (choose some file) -> tab Compare File -> Deployed Files
2. don't choose some system and click on Schedule File Comparison
3. Internal Server Error 

Actual results:
ISE 500

Expected results:
same message 

Additional info:
==> /var/log/tomcat6/catalina.out <==
2015-03-25 12:29:51,911 [TP-Processor12] WARN  org.apache.struts.action.RequestProcessor - Unhandled Exception thrown: class java.lang.NullPointerException
2015-03-25 12:29:51,911 [TP-Processor12] ERROR com.redhat.rhn.frontend.servlets.SessionFilter - Error during transaction. Rolling back
javax.servlet.ServletException: java.lang.NullPointerException
        at org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:520)
        at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:427)
        at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:228)
        at com.redhat.rhn.frontend.struts.RhnRequestProcessor.process(RhnRequestProcessor.java:102)
        at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913)
        at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:462)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at com.redhat.rhn.frontend.servlets.AuthFilter.doFilter(AuthFilter.java:127)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at com.opensymphony.sitemesh.webapp.SiteMeshFilter.obtainContent(SiteMeshFilter.java:129)
        at com.opensymphony.sitemesh.webapp.SiteMeshFilter.doFilter(SiteMeshFilter.java:77)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at com.redhat.rhn.frontend.servlets.LocalizedEnvironmentFilter.doFilter(LocalizedEnvironmentFilter.java:67)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at com.redhat.rhn.frontend.servlets.EnvironmentFilter.doFilter(EnvironmentFilter.java:100)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at com.redhat.rhn.frontend.servlets.SessionFilter.doFilter(SessionFilter.java:57)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at com.redhat.rhn.frontend.servlets.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:97)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
        at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:190)
        at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:291)
        at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:769)
        at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:698)
        at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:891)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:690)
        at java.lang.Thread.run(Thread.java:767)
Caused by:
java.lang.NullPointerException
        at com.redhat.rhn.manager.action.ActionManager.createConfigActionForServers(ActionManager.java:435)
        at com.redhat.rhn.manager.action.ActionManager.createConfigAction(ActionManager.java:514)
        at com.redhat.rhn.manager.action.ActionManager.createConfigDiffAction(ActionManager.java:409)
        at com.redhat.rhn.frontend.action.configuration.files.CompareDeployedSubmitAction.schedule(CompareDeployedSubmitAction.java:126)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:60)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
        at java.lang.reflect.Method.invoke(Method.java:611)
        at org.apache.struts.actions.DispatchAction.dispatchMethod(DispatchAction.java:269)
        at org.apache.struts.actions.DispatchAction.execute(DispatchAction.java:170)
        at org.apache.struts.actions.LookupDispatchAction.execute(LookupDispatchAction.java:146)
        at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:425)
        ... 40 more