NPE in ELFlash.get() on first request #4379

ELFlash.get throws NPE in clustered apps when trying to access flash scope on first request.

Reproducer: test.tar.gz

Steps to reproduce:

1. Unzip & deploy attached app
2. curl -i http://localhost:8080/test/getvar.xhtml

Result:
26-Jul-2018 11:06:59.383 SEVERE [http-nio-8080-exec-13] com.sun.faces.application.view.FaceletViewHandlingStrategy.handleRenderException Error Rendering View[/getvar.xhtml] javax.el.ELException: /getvar.xhtml: java.lang.NullPointerException at com.sun.faces.facelets.compiler.TextInstruction.write(TextInstruction.java:89) at com.sun.faces.facelets.compiler.UIInstructions.encodeBegin(UIInstructions.java:83) at com.sun.faces.facelets.compiler.UILeaf.encodeAll(UILeaf.java:211) at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1908) at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1908) at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:491) at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:194) at javax.faces.application.ViewHandlerWrapper.renderView(ViewHandlerWrapper.java:151) at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:126) at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:100) at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:223) at javax.faces.webapp.FacesServlet.service(FacesServlet.java:671) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:491) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:668) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:764) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1388) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:748) Caused by: java.lang.NullPointerException at com.sun.faces.context.flash.ELFlash.get(ELFlash.java:420) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at javax.el.BeanELResolver.invoke(BeanELResolver.java:158) at javax.el.CompositeELResolver.invoke(CompositeELResolver.java:79) at org.apache.el.parser.AstValue.getValue(AstValue.java:159) at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:190) at com.sun.faces.facelets.el.ELText$ELTextVariable.writeText(ELText.java:258) at com.sun.faces.facelets.el.ELText$ELTextComposite.writeText(ELText.java:165) at com.sun.faces.facelets.compiler.TextInstruction.write(TextInstruction.java:86) ... 33 more

Expected:
#{flash.get('var')} should return null since the variable has not been set.