Wicket form makes entire page disappear - forms

Question

A couple of years ago I was moderately proficient in Wicket. Today I picked it up again at the latest version, 6.12.0. I'm using it in an embedded Jetty server. I managed to configure everything and get a "Hello World" page working.
But here's the odd thing: the minute I add a form, the entire page disappears! Let's say I have:
<h1>Hello</h1>
<p>blah</p>
<p><label>Upload File:</label> <input wicket:id="fileUpload"
type="file" size="40" /><br /> <input wicket:id="uploadButton"
type="submit" value="Upload" /></p>
That works fine. Then I add:
<h1>Hello</h1>
<p>blah</p>
<form wicket:id="form">...</form>
Suddenly the page is blank! The response code is still 200 OK, but there is no content. Here's what comes back:
Date: Tue, 19 Nov 2013 01:32:48 GMT
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Pragma: no-cache
Cache-Control: no-cache, no-store
Transfer-Encoding: chunked
Server: Jetty(9.1.0.v20131115)
200 OK
Yes, when I add the form, I make sure the components on the form (e.g. the buttons) are now programmatically added to the form instead of the page:
final Form form=new Form("form");
form.add(new FileUploadField("fileUpload"));
form.add(new Button("uploadButton"));
add(form);

Answer 1

My Jetty configuration had disabled sessions. I didn't see the exception (which was clear about the problem) because I hadn't configured log4j for the project. After changing NO_SESSIONS to SESSIONS forms seem to work fine:
final ServletContextHandler contextHandler = new ServletContextHandler(ServletContextHandler.SESSIONS);