Visual composer custom element error

I am creating a custom element where I need to load woo-commerce product categories as dropdown (in visual composer element edit option)

but I am getting this error. can someone tell me where is the problem?

here is the code:

?php
add_shortcode('woo_my_cat_sub_cat_show_element_code','woo_my_cat_sub_cat_show_element_functions');
function woo_my_cat_sub_cat_show_element_functions($atts,$content){
    extract(shortcode_atts(array(
        'woo_my_cat_list' =  '',
    ),$atts));

    ob_start();?

        !-- I will do the frontend here --
        
    ?php 
    return ob_get_clean();


}


if(function_exists('vc_map')){
   /***
   * dynamic fields for the element
   */
    $args = array(
        'taxonomy'   = 'product_cat',
        // 'number'     = $number,
        // 'orderby'    = $orderby,
        // 'order'      = $order,
        // 'hide_empty' = $hide_empty,
        //'include'    = 59,
        'parent'    =  0,
    );
    $product_categories = get_terms($args);

    $dropdown_cats_array = array();
    foreach( $product_categories as $single ){
        $dropdown_cats_array[$single-name] = $single-term_id; 
    }
        
    vc_map(array(

        'name' = 'Subcategory',
        'base' = 'woo_my_cat_sub_cat_show_element_code',
        'category'  = 'Custom Element',
        'params' = array(
            array(
                'param_name' = 'woo_my_cat_list',
                'heading' = 'Select Category',
                'type' = 'dropdown',
                'value' =  $dropdown_cats_array,
                description = Please Select the category, which subcategories you want to show on the page.,
            ),

        )

    ));
}
?

And here is the error I am getting: