undertow + Spring WebApplicationInitializer - Double initialization

Question

I am migrating an old web app. It was written with old web.xml-style, now we want to programmatically build the servlet.

This is a Spring MVC app, deployed as a WAR module in an EAR under Wildfly 10.0.0.Final.

I wrote my implementation of Spring's WebApplicationInitializer (still using Spring XML config at the moment, to preceed step by step - next step will be to migrate to JavaConfig). Though I'm stuck here because the servlet is initialized twice, and the second time the initilization fails due to a filter's name conflict - NullPointerException after filter creation.

This is my initilizer:

public class MyWebApplicationInitializer implements WebApplicationInitializer {
    private static final Logger log = LoggerFactory.getLogger(MyWebApplicationInitializer.class);

    /**
     * @see org.springframework.web.WebApplicationInitializer#onStartup(javax.servlet.ServletContext)
     */
    @Override
    public void onStartup(ServletContext container) throws ServletException {
        log.debug("******* Initializing Web App *******");
        XmlWebApplicationContext rootContext = new XmlWebApplicationContext();
        rootContext.setConfigLocation("classpath:META-INF/spring/application-context.xml");

        XmlWebApplicationContext webContext = new XmlWebApplicationContext();
        webContext.setConfigLocation("classpath:META-INF/spring/mvc-context.xml");

        container.addListener(new ContextLoaderListener(rootContext));

        ServletRegistration.Dynamic dispatcher = container.addServlet("dispatcher", new DispatcherServlet(webContext));
        dispatcher.setLoadOnStartup(1);
        dispatcher.addMapping("/");

        container.addFilter("log4jEnhancementFilter", Log4jEnhancementFilter.class);

        log.info("******* Web App correctly initialized *******");
    }

}

A bit of log below:

[ServerService Thread Pool -- 74]:[INFO] @ 2016-04-19 07:19:27,725: io.undertow.servlet.spec.ServletContextImpl.log:313: Spring WebApplicationInitializers detected on classpath: [com.xyz.web.servlet.MyWebApplicationInitializer@6356dd91]
[ServerService Thread Pool -- 74]:[DEBUG] @ 2016-04-19 07:19:27,725: com.xyz.web.servlet.MyWebApplicationInitializer.onStartup:44: ******* Initializing Web App *******
...
[ServerService Thread Pool -- 74]:[INFO] @ 2016-04-19 07:19:27,796: com.xyz.web.servlet.MyWebApplicationInitializer.onStartup:59: ******* Web App correctly initialized *******
...
[ServerService Thread Pool -- 74]:[INFO] @ 2016-04-19 07:19:27,797: io.undertow.servlet.spec.ServletContextImpl.log:313: Spring WebApplicationInitializers detected on classpath: [com.xyz.web.servlet.MyWebApplicationInitializer@427d3c64]
[ServerService Thread Pool -- 74]:[DEBUG] @ 2016-04-19 07:19:27,797: com.xyz.web.servlet.MyWebApplicationInitializer.onStartup:44: ******* Initializing Web App *******
07:19:27,800 ERROR [org.jboss.msc.service.fail] (ServerService Thread Pool -- 74) MSC000001: Failed to start service jboss.undertow.deployment.default-server.default-host./www: org.jboss.msc.service.StartException in service jboss.undertow.deployment.default-server.default-host./www: java.lang.RuntimeException: java.lang.NullPointerException
    at org.wildfly.extension.undertow.deployment.UndertowDeploymentService$1.run(UndertowDeploymentService.java:85)
    at java.util.concurrent.Executors$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)
    at org.jboss.threads.JBossThread.run(JBossThread.java:320)
Caused by: java.lang.RuntimeException: java.lang.NullPointerException
    at io.undertow.servlet.core.DeploymentManagerImpl.deploy(DeploymentManagerImpl.java:231)
    at org.wildfly.extension.undertow.deployment.UndertowDeploymentService.startContext(UndertowDeploymentService.java:100)
    at org.wildfly.extension.undertow.deployment.UndertowDeploymentService$1.run(UndertowDeploymentService.java:82)
    ... 6 more
Caused by: java.lang.NullPointerException
    at com.xyz.web.servlet.MyWebApplicationInitializer.onStartup(MyWebApplicationInitializer.java:54)
    at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:175)
    at io.undertow.servlet.core.DeploymentManagerImpl.deploy(DeploymentManagerImpl.java:184)
    ... 8 more

As you can see there is a double servlet's initilization; the first succeeds, the second fails due to the NPE.

What am I doing wrong and how can I fix it?