Can’t save metadata in custom plugin with custom metabox?

With the tutorial series of codeacademy.cc(https://www.youtube.com/watch?v=33bilBgZrb4) I tried to create the following plugin. However, it doesn’t save the metadata. I don’t know what is happening here and I can’t find out why it is not saving it. I put different “die” functions into the if clauses for a kind of debug. It seems that $_POST['the-key'] is not set. When I try to use an else clause to add_meta_data, it also changes nothing. Does anyone have a clue how I can fix this code?

Also I tried different hooks:

• pre_post_update
• save_post
• post_updated

(adding a line because stackoverflow has a problem with list+code)

<?php
/*
* Plugin Name: Zenva Video Widget
* Plugin URI: http://www.zenva.com
* Description: Learn how to create shortcodes and to retrieve data from the web.
* Version: 1.0
* Author: IQ Development
* Author URI: http://www.zenva.com
* License: GPL2
*/

add_action('add_meta_boxes', 'zvavw_add_metabox');
add_action('save_post', 'zvavw_save_metabox');


function zvavw_add_metabox() {
    add_meta_box('zvavw_youtube', 'YouTube Video Link', 'zvavw_youtube_handler', 'post');
}

function zvavw_youtube_handler() {
    $value = get_post_custom($post->ID);
    $youtube_link = esc_attr($value['zvavw_youtube'][0]);

    echo '<label for="zvavw_youtube">YouTube Video Link</label><input type="text" id="zvavw_youtube" value="' . $youtube_link . '" />';
}

function zvavw_save_metabox($post_id) {

    if(defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return;
    }

    if(!current_user_can('edit_post')) {
        return;
    }

    if(isset($_POST['zvavw_youtube'])) {
        update_post_meta($post_id, 'zvavw_youtube', esc_url($_POST['zvavw_youtube']));
    }
}

?>