I've looked at previous examples of this problem but I can't find a solution for what is causing this notice based on what I've seen from other people.

1145 is:

    $post = get_post( $args[0] );

I'm not getting any other kind of error so I'm not sure where in my code this is causing the problem.

**Any help on this?**

This is the code:

    //show metabox in post editing page
    add_action('add_meta_boxes', 'kk_add_metabox' );

    //save metabox data
    add_action('save_post', 'kk_save_metabox' );

    //register widgets
    add_action('widgets_init', 'kk_widget_init');

    function kk_add_metabox() {
        add_meta_box('kk_youtube', 'YouTube Video Link','kk_youtube_handler', 'post');
    }

    /**
     * metabox handler
     */
    function kk_youtube_handler($post)
    {
        $youtube_link = esc_attr( get_post_meta( $post->ID, 'kk_youtube', true ) );

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

    /**
     * save metadata
     */
    function kk_save_metabox($post_id) {
    
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    //check if user can edit post
    if( !current_user_can( 'edit_post' ) ) {
        return;
    }

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

    /**
     * register widget
     */
    function kk_widget_init() {
        register_widget('KK_Widget');
    }

    /**
     * Class KK_Widget widget class
     */
    class KK_Widget extends WP_Widget
    {
    function __construct()
    {
        $widget_options = array(
            'classname' => 'kk_class', // For CSS class name
            'description' => 'Show a YouTube video from post metadata'
        );

        $this->WP_Widget('kk_id', 'YouTube Video', $widget_options);
    }

    /**
     * Show widget form in Appearance/Widgets
     */
    function form($instance)
    {
        $defaults = array(
            'title' => 'YouTube Video'
        );

        $instance = wp_parse_args((array)$instance, $defaults);
        var_dump($instance);
        $title = esc_attr($instance['title']);

        echo '<p>Title <input type="text" class="widefat" name="' . $this->get_field_name('title') . '" value="' . $title . '" /></p>';
    }

    function update($new_instance, $old_instance)
    {
        $instance = $old_instance;

        $instance['title'] = strip_tags($new_instance['title']);

        return $instance;
    }

    /**
     * Show widget in post/page
     */
    function widget($args, $instance)
    {
        global $before_widget;
        global $after_widget;
        global $before_title;
        global $after_title;

        extract( $args );
        $title = apply_filters('widget_title', $instance['title']);

        //show only if single post
        if(is_single()) {
            echo $before_widget;
            echo $before_title.$title.$after_title;

            //get post metadata
            $kk_youtube = esc_url(get_post_meta(get_the_ID(), 'kk_youtube', true));

            //print widget content
            echo '<iframe width="200" height="200" frameborder="0" allowfullscreen src="http://www.youtube.com/embed/' . $this->get_yt_videoid($kk_youtube) . '"></iframe>';

            echo $after_widget;
        }
    }

    function get_yt_videoid($url)
    {
        parse_str(parse_url($url, PHP_URL_QUERY), $my_array_of_vars);

        return $my_array_of_vars['v'];
    }
    }