show only first level in taxonomy level drupal 8

In this tuto, I'll show you how to display only first level in taxonomy level drupal 8.

Default select with 2 levels:

Example how to display  only first level in taxonomy level drupal 8.

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id)
{
    if ($form_id == 'node_article_edit_form') {
        $options_category = $form['field_category']["widget"]['#options'];
        foreach ($options_category as $key => $category) {
            if (isset($category) && !is_object($category)) {
                if (strpos($category, '-', 0) === 0 && $key != '_none') {
                    unset($options_category[$key]);
                }
            }
        }
        $form['field_category']["widget"]['#options'] = $options_category;
    }
}

The result is like this: