Can't use the variable $checkout with WooCommerce-Multistep-Checkout - php

Question

I'm trying to create a new step in my checkout page and i'm currently using the plugin WooCommerce-Mulitstep-Checkout plugin. I followed the documentation to create a new step (here : http://woocommerce-multistep-checkout.com/documentation/). So here is my code in function.php:
add_action('woocommerce_multistep_checkout_before_order_info', 'add_my_custom_step_with_new_field');
function add_my_custom_step_with_new_field( $checkout ) {
wc_get_template( 'checkout/my-custom-step.php', array( 'checkout' => $checkout, 'test' => "test" ) );
}
add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );
function my_custom_checkout_field_update_order_meta( $order_id ) {
if ( ! empty( $_POST['my_field_name'] ) ) {
update_post_meta( $order_id, 'My Field', sanitize_text_field( $_POST['my_field_name'] ) );
}
}
And here is my code in checkout/my-custom-step.php :
<h1> Step Title</h1>
<div class="my-custom-step">
<?php
woocommerce_form_field('my_field_name', array(
'type' => 'text',
'required' => true,
'class' => array('my-field-class form-row-wide'),
'label' => __('Fill in this field'),
'placeholder' => __('Enter something'),
), $checkout->get_value('my_field_name'));
?>
</div>
}
When I use this code (mostly provided by the official documentation), I get this error on the browser console :
GET https://localhost/mywebsite/checkout/ 500 (Internal Server Error)
And when I remove , $checkout->get_value('my_field_name'), it works perfectly fine (but without prefilling the fields.
Moreover, when I use var_dump($checkout);, it displays me this : string(0) "".
Finally, when I want to display $test, it works perfectly fine. So I don't think I use badly the variable.
So my questions are :
Do you know all the mystery behind this empty variable ?
Did I misused something ?
Why is there a final curly bracket at the end of the template in the official documentation ?
Thanks