I am creating a dynamic Block to display a widget area inside post. I coded the following in ES5. The block was registered successfully, but it has failed to render the I want. I have looked up a lot of tutorials, but I am not sure what I have missed.

php code:

    function registerWidgetBlockArea() {

    register_sidebar( array(
    'name' => esc_html__( 'Widget Area for Widget Block', 'survivalisthk' ),
    'id' => 'widget-block-area',
    'description'   => '',
    'before_widget' => '<aside id="%1$s" class="widget %2$s">',
    'after_widget'  => '</aside>',
    'before_title'  => '<h2 class="widget-title">',
    'after_title'   => '</h2>',
)); 

}
add_action( 'widgets_init', 'registerWidgetBlockArea');

//callback function for Widget Block
function createWidgetArea($attributes){
    if ( is_active_sidebar( 'widget-block-area' ) ) {
        echo '<div id="mobile-menu-widget-area" class="mobile-menu-widget widget-area" role="complementary">';
        dynamic_sidebar( 'widget-block-area' );
        echo '</div>';
    }
    echo 'Testing from callback function';
    return 'Testing from callback function';

}

add_action('init', 'registerCustomBlocks');

function registerCustomBlocks() {
    //Widget block for displaying widget
    $block_path2 = '/widget_block.js';
    wp_register_script(
        'CUSTOM-block-widget',
        plugins_url( $block_path2 , __FILE__ ),
        [ 'wp-i18n', 'wp-element', 'wp-blocks', 'wp-components', 'wp-editor' ],
        filemtime( plugin_dir_path( $block_path2 , __FILE__ ) )
    );

    register_block_type( 'custom-blocks/widget', array(
        'editor_script' => 'CUSTOM-block-widget',
        //'render_callback' => 'createWidgetArea',
    ) );

}

JS:

wp.blocks.registerBlockType( 'custom-blocks/widget', {

    title: 'Widget Area',

    icon: 'megaphone',

    category: 'custom_block',

    support: {
        html: true,
        reusable: false,
    },

    edit: function( props ) {
           return (
                el( ServerSideRender, {
                    block: 'custom-blocks/widget',
                    attributes: props.attributes,
                } )
            );
    },

    save: function( props ) {
        return null;
    }
} );

Thank you!