Make the error message to be more informative

[Artur-]

Describe the bug
If you configure a starter project from start.vaadin.com to produce a war and deploy it to Tomcat 8, you get

25-Feb-2021 10:16:32.356 INFO [localhost-startStop-3] org.apache.catalina.core.ApplicationContext.log 1 Spring WebApplicationInitializers detected on classpath
25-Feb-2021 10:16:32.363 INFO [localhost-startStop-3] org.apache.catalina.core.ApplicationContext.log Initializing AtmosphereFramework
25-Feb-2021 10:16:32.698 SEVERE [localhost-startStop-3] org.apache.catalina.core.StandardContext.listenerStart Exception sending context initialized event to listener instance of class [com.vaadin.flow.server.startup.ServletContextListeners]
        java.lang.NullPointerException
                at com.vaadin.flow.server.startup.ApplicationConfiguration.lambda$get$0(ApplicationConfiguration.java:52)
                at com.vaadin.flow.server.startup.ApplicationConfiguration$$Lambda$14512/13359214.get(Unknown Source)
                at com.vaadin.flow.server.VaadinServletContext.getAttribute(VaadinServletContext.java:73)
                at com.vaadin.flow.server.startup.ApplicationConfiguration.get(ApplicationConfiguration.java:49)
                at com.vaadin.flow.server.DeploymentConfigurationFactory.createPropertyDeploymentConfiguration(DeploymentConfigurationFactory.java:82)
                at com.vaadin.flow.server.startup.ServletDeployer$StubServletConfig.createDeploymentConfiguration(ServletDeployer.java:184)
                at com.vaadin.flow.server.startup.ServletDeployer.lambda$getServletConfigurations$0(ServletDeployer.java:291)
                at com.vaadin.flow.server.startup.ServletDeployer$$Lambda$14511/25690887.accept(Unknown Source)
                at java.util.Optional.ifPresent(Optional.java:159)
                at com.vaadin.flow.server.startup.ServletDeployer.getServletConfigurations(ServletDeployer.java:290)
                at com.vaadin.flow.server.startup.ServletDeployer.contextInitialized(ServletDeployer.java:208)
                at com.vaadin.flow.server.startup.ServletContextListeners.contextInitialized(ServletContextListeners.java:42)
                at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4699)
                at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5165)
                at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
                at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:743)
                at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:719)
                at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:714)
                at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:970)
                at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:1841)
                at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
                at java.util.concurrent.FutureTask.run(FutureTask.java:266)
                at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
                at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
                at java.lang.Thread.run(Thread.java:745)

To Reproduce
Steps to reproduce the behavior:

git clone https://github.com/Artur-/test-war-deployment.git
cd test-war-deployment
mvn clean install -Pproduction
cp target/*.war tomcat/webapps/

Expected behavior
The app is deployed.

Versions

• Tomcat: 8.5.46 on Linux
• Vaadin platform version: 19.0.0.beta4

[Artur-]

Ok, this seems to be an issue when you build a project using Java 11 and deploy it to a server running with Java 8.

If you do this with Vaadin 18 or Vaadin 19 alpha 1, you see

10:39:57.426 [localhost-startStop-6] INFO com.vaadin.flow.server.startup.ServletDeployer - Skipping automatic servlet registration because there are no navigation targets registered to the route registry and there are no web component exporters.

which is not very informative either but at least not a stack trace indicating something is wrong in Vaadin.

Can we have a better error message?