CMB2 show_on Filter for Default Page Template

cmb2_metabox_include_default_page.php

<?php
/**
* Include metabox only on the default page template (page.php). Heavily based of Ed Townend's front-page solution
* @author Rob Clark
*
* @param bool $display
* @param array $meta_box
* @return bool display metabox
*/
function cmb2_metabox_include_default_page( $display, $meta_box ) {
    if ( ! isset( $meta_box['show_on']['key'] ) ) {
      return $display;
    }

    if ( 'default-page-template' !== $meta_box['show_on']['key'] ) {
      return $display;
    }

    $post_id = 0;

    // If we're showing it based on ID, get the current ID
    if ( isset( $_GET['post'] ) ) {
      $post_id = $_GET['post'];
    } elseif ( isset( $_POST['post_ID'] ) ) {
      $post_id = $_POST['post_ID'];
    }

    if ( ! $post_id ) {
      return false;
    }

    $page_template = get_page_template_slug( $post_id );

    if ( empty($page_template) ) {
        $is_it_basic = true;
    } else {
        $is_it_basic = false;	
    }

    // there is a front page set and we're on it!
    return $is_it_basic;
}
add_filter( 'cmb2_show_on', 'cmb2_metabox_include_default_page', 10, 2 );

/**
* Example Usage:
*
* $cmb_page_default_options = new_cmb2_box( array(
*     'id'            => $prefix . 'metabox',
*     'title'         => __( 'Default Page Template Options', 'mytheme' ),
*     'object_types'  => array( 'page' ),
*     'show_on' => array( 'key' => 'default-page-template', 'value' => '' ),
* ) );
*
*/