ExtJs: autoCreateViewport not working?

ExtJs: autoCreateViewport not working?

I'm following the Sencha guide exactly, but it seems that
autoCreateViewport isn't working. Nothing renders, launch isn't called,
and I have no warning or errors.
app.js
Ext.application({
name: 'PT',
autoCreateViewport: true,
launch: function() {
console.log('launched'); // never called
}
});
extjs/view/Viewport.js
Ext.define('PT.view.Viewport', {
extend: 'Ext.container.ViewPort',
layout: 'fit',
items: [ { title: 'Hello World', html: 'some text..'} ],
initComponent: function() {
this.callParent();
}
});
Please help. I'm so close to going back to Dojo.