WP_List_Table не отоброжает данные

<?php
if( !class_exists('WP_List_Table')){
    require_once ( ABSPATH . 'wp-admin/includes/class-wp-list-table.php' );
}
class My_List_Table extends WP_List_Table{
    public $example_data =
        array(
            array('ID' => 1,'booktitle' => 'Quarter Share', 'author' => 'Nathan Lowell',
                'isbn' => '978-0982514542'),
            array('ID' => 2, 'booktitle' => '7th Son: Descent','author' => 'J. C. Hutchins',
                'isbn' => '0312384378'),
            array('ID' => 3, 'booktitle' => 'Shadowmagic', 'author' => 'John Lenahan',
                'isbn' => '978-1905548927'),
            array('ID' => 4, 'booktitle' => 'The Crown Conspiracy', 'author' => 'Michael J. Sullivan',
                'isbn' => '978-0979621130'),
            array('ID' => 5, 'booktitle'     => 'Max Quick: The Pocket and the Pendant', 'author'    => 'Mark Jeffrey',
                'isbn' => '978-0061988929'),
            array('ID' => 6, 'booktitle' => 'Jack Wakes Up: A Novel', 'author' => 'Seth Harwood',
                'isbn' => '978-0307454355')
        );
    function get_columns(){
        $columns = array(
            'booktitle' => 'Title',
            'author'    => 'Author',
            'isbn'      => 'ISBN'
        );
        return $columns;
    }
    function prepare_items() {
        $columns = $this->get_columns();
        $hidden = array();
        $sortable = array();
        $this->_column_headers = array($columns, $hidden, $sortable);
        $this->items = $this->example_data;
    }

}
$myListTable = new My_List_Table();
echo '<div class="wrap">
            <div id="icon-users" class="icon32"></div>
                <h2>My List Table Test</h2>
                                        ';
$myListTable->prepare_items();
$myListTable->display();
echo '</div>';

Выводит:

В чем ошибка? Почему выводит пустую таблицу?