gistfile1.php

/**
 * Fix page template issue causing metaboxes not to save
 */
add_action('admin_init','generate_update_page_template_issue');
function generate_update_page_template_issue()
{
    global $post;
                
    $args = array(
        'post_status' => 'publish',
        'posts_per_page' => -1,
        'post_type' => 'page'
    );
    $posts = get_posts($args);
    foreach ( $posts as $post ) {
        $page_template = $post->page_template;
        if( !locate_template($page_template) ) :
            update_post_meta($post->ID, '_wp_page_template', 'default');
        endif;
    }
}