custom widget: modal_group items not loading after page is saved

/**
* Create the config array for the shortcode button
*/
function shortcode_insert_button()
{
$this->config['name'] = __('Merken (lijst)', 'vdlp');
$this->config['tab'] = __('VDLP', 'vdlp');
$this->config['icon'] = AviaBuilder::$path['imagesURL']."sc-blog.png";
$this->config['order'] = 20;
$this->config['target'] = 'avia-target-insert';
$this->config['shortcode'] = 'vdlp_sc_merken_list';
$this->config['shortcode_nested'] = array('vdlp_sc_merken_list_merk');
$this->config['tooltip'] = __('Lijst van merken', 'vdlp');
}

/**
* Popup Elements
*
* If this function is defined in a child class the element automatically gets an edit button, that, when pressed
* opens a modal window that allows to edit the element properties
*
* @return void
*/
function popup_elements() {

global $merken_module;

$posts = $merken_module->queryItems();
if (!empty($posts)) {
foreach ($posts as $post) {
$merken[get_field('merk_naam', $post->ID)] = $post->ID;
}
}

$this->elements = array(
array(
"name" => __("Toevoegen/wijzigen lijst met merken", 'vdlp' ),
"desc" => __("Hier kunt u merken toevoegen, wijzigen of verwijderen uit uw lijst", 'vdlp' ),
"type" => "modal_group",
"id" => "content",
"modal_title" => __("Merk", 'vdlp'),
"std" => array(),
'subelements' => array(
array(
"name" => __("Titel", 'vdlp' ),
"desc" => __("Geef titel op", 'vdlp' ),
"id" => "title",
"type" => "input",
),
array(
"name" => __("Kies gekoppelde merk", 'vdlp' ),
"desc" => __("Selecteer een merk uit de lijst", 'vdlp' ),
"id" => "merkid",
"type" => "select",
"subtype" => $merken,
),
)
)
);
}

/**
* Editor Element - this function defines the visual appearance of an element on the AviaBuilder Canvas
* Most common usage is to define some markup in the $params['innerHtml'] which is then inserted into the drag and drop container
* Less often used: $params['data'] to add data attributes, $params['class'] to modify the className
*
*
* @param array $params this array holds the default values for $content and $args.
* @return $params the return array usually holds an innerHtml key that holds item specific markup.
*/
function editor_element($params)
{
$params['innerHtml'] = "<img src='".$this->config['icon']."' title='".$this->config['name']."' />";
$params['innerHtml'].= "<div class='avia-element-label'>".$this->config['name']."</div>";
$params['content'] = NULL; //remove to allow content elements

return $params;
}

/**
* Editor Sub Element - this function defines the visual appearance of an element that is displayed within a modal window and on click opens its own modal window
* Works in the same way as Editor Element
* @param array $params this array holds the default values for $content and $args.
* @return $params the return array usually holds an innerHtml key that holds item specific markup.
*/
function editor_sub_element($params)
{
$template = $this->update_template("title", "{{title}}");

$params['innerHtml'] = "<div class='avia_title_container'>";
$params['innerHtml'] .= "<span {$template}>" .$params['args']['title']. "</span>";
$params['innerHtml'] .= "</div>";

return $params;
}

/**
* Frontend Shortcode Handler
*
* @param array $atts array of attributes
* @param string $content text within enclosing form of shortcode element
* @param string $shortcodename the shortcode found, when == callback name
* @return string $output returns the modified html string
*/
function shortcode_handler($atts, $content = "", $shortcodename = "", $meta = "")
{
$output = "";

$atts = shortcode_atts(array(
'items' => 0,
), $atts, $this->config['shortcode']);

$merken = ShortcodeHelper::avia_remove_autop($content, true);

if (empty($merken)) {
return '';
}

$output .= '<div class="vdlp-widget-wrapper">';
$output .= '<h2>Merken</h2>';
$output .= '<ul class="merken">';
$output .= ShortcodeHelper::avia_remove_autop( $content, true );
$output .= '</ul>';
$output .= '<a href="' . get_permalink(CONST_PAGEID_MERKEN) . '" title="Meer merken">Meer merken</a>';
$output .= '</div>';

return $output;
}

/**
* vdlp_sc_merken_list_merk
*
* @param array $atts
* @param string $content
* @param string $shortcodename
* @return string
*/
function vdlp_sc_merken_list_merk($atts, $content = "", $shortcodename = "")
{
$atts = shortcode_atts(array(
'merkid' => '',
'title' => ''
), $atts, $this->config['shortcode_nested']);

$atts['title'] = (empty($atts['title'])) ? get_field('merk_naam', $atts['merkid']) : $atts['title'];

$output = '<li><a title="Ga naar '. $atts['title'] .'" href="' . get_permalink($atts['merkid']) . '"><span>' . $atts['title'] .'</span></a></li>';