NPE in wicket 7.1.0

HomePage.java

package com.mycompany.myproject;

import org.apache.wicket.ajax.AbstractAjaxTimerBehavior;
import org.apache.wicket.ajax.AjaxRequestTarget;
import org.apache.wicket.devutils.debugbar.DebugBar;
import org.apache.wicket.markup.html.WebPage;
import org.apache.wicket.markup.html.basic.Label;
import org.apache.wicket.request.mapper.parameter.PageParameters;
import org.apache.wicket.util.time.Duration;

public class HomePage extends WebPage {
	private static final long serialVersionUID = 1L;
	private final DebugBar dev;

	public HomePage(final PageParameters parameters) {
		super(parameters);

		add(new Label("version", getApplication().getFrameworkSettings().getVersion()));
	    add(dev = new DebugBar("dev"));
	    dev.setOutputMarkupId(true);
		// TODO Add your page's components here
		add(new AbstractAjaxTimerBehavior(Duration.ONE_SECOND) {
			private static final long serialVersionUID = 1L;

			@Override
			protected void onTimer(AjaxRequestTarget target) {
				target.add(dev);
			}
		});
    }
}