使用collectionType的symfony 4错误原型

Question

我正在使用symfony 4，并且尝试使用集合类型以便能够添加相同实体的许多对象。我遵循了本指南，但收到以下错误：

Key "prototype" for array with keys "value, attr, form, id, name, full_name, disabled, label, label_format, multipart, block_prefixes, unique_block_prefix, translation_domain, cache_key, errors, valid, data, required, size, label_attr, compound, method, action, submitted, sonata_admin_enabled, sonata_help, sonata_admin, horizontal_label_class, horizontal_label_offset_class, horizontal_input_wrapper_class" does not exist.

实际上，当我打印数组时，'prototype'键不存在，类似的问题here并没有帮助我。我收到同样的错误。 到目前为止，这是我尝试过的：

ApplicationType.php

   $builder->add('previousjobs', CollectionType::class, array(
    'entry_type' => WorkExperienceType::class,
    'allow_add' => true,
    'prototype' => true,
    'prototype_data' => 'New Tag Placeholder',
))

Application.php

public $previousjobs;

public function getPreviousjobs()
 {
     return $this->previousjobs;
 }

form.html.twig

    <ul class="tags">
        {# iterate over each existing tag and render its only field: name #}
         {% for tag in form.previousjobs %}
             {# {{ dump(tag.vars.name)|e }}  #}
               {# {{ form_widget(tag.vars.name)|e }}  #}
              {# {{form_row(tag.vars.name)}}  #}
             {# {{ dump(tag)}} #}
        {% endfor %}
    </ul>
{{ form_end(form) }}

<ul class="tags" data-prototype="{{ form_widget(form.previousjobs.vars.prototype)|e('html_attr') }}">
    ...
</ul>

非常感谢任何帮助。