Symfony-twig template form theme error

I have everything working correctly and now I'm trying to work with form themes. This is my code to generate the form without a theme.

{% extends 'base.html.twig' %}
{% block body %}
{% include 'menu/menu.html.twig' %}
{% if addpost is defined %}
    <div id='add_post_form'>
        {{ form_start(addpost) }}
        {{ form_widget(addpost) }}
        {{ form_end(addpost) }}
    </div>
{% endif %}
{% endblock %}

But when I'm adding a form-theme with the following code

{% form_theme form 'form/form_div_layout.html.twig' %}

I get this error:

Variable "form" does not exist

When i execute this without the line, I'm getting the following error:

Unknown "form_help" function. Did you mean "form_rest", "form_end"?

the form_div_layout.html.twig contains the code found at github symfony twig form theme

At my config.yml I've added the following under the twig section,

form_themes:
        - 'form/form_div_layout.html.twig'

.

either not, i still have this error what is missing ???

---

My file structure

If all your forms are going to use the same theme you only need to add the line in your config, but if you want a particular form theme in a particular template you can use the template tag.

The reason you're getting the 'form is not defined error' is because you don't have a variable called form passed the the template, your form variable is called addpost, so you need to use

{% form_theme addpost 'form/form_div_layout.html.twig' %}