With the form factory there was no reasonable way to implement instantiation of custom form classes. So the implementation was changed to let the classes instantiate themselves. A FormContext instance with default settings has to be passed to the creation method. This context is by default configured in the DI container.
$context = $this->get('form.context');
// or
$context = FormContext::buildDefault();
$form = MyFormClass::create($context, 'author');
If you want to circumvent this process, you can also create a form manually. Remember that the services stored in the default context won't be available then unless you pass them explicitely.
$form = new MyFormClass('author');
