Symfony 3 Dynamic Generation for Submitted Forms not working

I'm following the tutorial at http://symfony.com/doc/current/form/dynamic_form_modification.html#dynamic-generation-for-submitted-forms

However every attempt gives me this error. Note: Real bundle renamed for AcmeBundle.

Catchable Fatal Error: Argument 1 passed to
AcmeBundle\Form\ScheduleType::AcmeBundle\Form\{closure}()
must be an instance of
AcmeBundle\Form\FormInterface,
instance of Symfony\Component\Form\Form given,
called in
/var/www/acme/api/src/AcmeBundle/Form/ScheduleType.php on line 52 and defined 

/var/www/acme/api/src/AcmeBundle/Form/ScheduleType.php

<?php

namespace AcmeBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type\CollectionType;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;

// form input types
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\Extension\Core\Type\CheckboxType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use Symfony\Component\Form\Extension\Core\Type\TimeType;

//use AcmeBundle\Entity\Classes;
//use AcmeBundle\Entity\Subject;

class ScheduleType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {

        $builder->add('Classes',
                          EntityType::class,
                          array('class'         => 'AcmeBundle:Classes',
                                'expanded'      =>  false,
                                'placeholder' => '',
                                'multiple'      =>  false,
                                'label'         =>  'Class'));
 35
        $formModifier = function (FormInterface $form, Classes $classes = null) {
            $subjects = null === $classes ? array() : $classes->getSubjects();

            $form->add('position', EntityType::class, array(
                'class'       => 'AppBundle:Subject',
                'placeholder' => '',
                'choices'     => $subjects,
            ));
        };

        $builder->addEventListener(
            FormEvents::PRE_SET_DATA,
            function (FormEvent $event) use ($formModifier) {
                // this would be your entity, i.e. SportMeetup
                $data = $event->getData();

                $formModifier($event->getForm(), $data->getClasses());
            }
        );

        $builder->get('Classes')->addEventListener(
            FormEvents::POST_SUBMIT,
            function (FormEvent $event) use ($formModifier) {
                $classes = $event->getForm()->getData();
                $formModifier($event->getForm()->getParent(), $classes);
            }
        );
    }
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AcmeBundle\Entity\Schedule',
        ));
    }

}

src/AcmeBundle/Controller/DefaultController.php

public function addScheduleAction( Request $request )
{
    $sche = new Schedule();

    $form = $this->createForm(ScheduleType::class, $sche);
    $form->handleRequest($request);

    if( $form->isSubmitted() && $form->isValid() )
    {
        // form saving
    }

    return $this->render(   'AcmeBundle:schedule:layout_schedule_form.html.twig',
                            array(  'title'     => $translator->trans('%AddNewSchedule%'),
                                    'form'      =>  $form->createView(),
                                    'message'   => '')
    );
}

src/AcmeBundle/Resources/views/schedule/layout_schedule_form.html.twig

{% extends "AcmeBundle::layout.html.twig" %}

{% block cmi_body %}

    <div class="row">
        <div class="col-md-12">
            <p>{{ message }}</p>
        </div>
    </div>
    <div class="row">
        <div class="col-md-2">

        </div>
        <div class="col-md-6">
            {{form_start(form)}}
            {{ form_row(form.Classes) }}    {# <select id="schedule_classes" ... #}
            {{ form_row(form.Subjects) }} {# <select id="schedule_subjects" ... #}
            {{form_end(form)}}
        </div>
    </div>
<script>
var $classes = $('#schedule_classes');
$classes.change(function() {
  var $form = $(this).closest('form');
  var data = {};
  data[$classes.attr('name')] = $classes.val();
  $.ajax({
    url : $form.attr('action'),
    type: $form.attr('method'),
    data : data,
    success: function(html) {
      $('#schedule_subjects').replaceWith(
        $(html).find('#schedule_subjects')
      );
 }
  });
});
</script>
{% endblock %}