Implement custom-fields library class

class-widget-custom-fields.php

<?php
/**
 * Widget custom fields class
 *
 * Extends Custom_Fields class
 *
 * @since 1.0.0
 * @author Stellar Blue Technologies
 * @licence GNU-2.0+
 */

/**
 * Class Widget custom fields
 */
class Widget_Custom_Fields extends Custom_Field {

	/**
	 * Hold instance of class
	 *
	 * @var $instance
	 */
	public static $instance;

	/**
	 * @var $widget_prefix
	 */
	public $widget_prefix = 'widget_';

	/**
	 * @var array $widget_meta_fields
	 */
	public $widget_meta_fields = [];

	/**
	 * Widget Custom Fields constructor.
	 *
	 * @since 1.0.0
	 */
	public function __construct() {


	}

	/**
	 * Get custom meta fields
	 *
	 * Define the custom fields for the CPT
	 *
	 * @since 1.0.0
	 * @return array $custom_meta_fields
	 */
	public function get_widget_meta_fields() {

		$prefix = $this->widget_prefix;

		$this->licensee_meta_fields = array(
			array(
				'label' => 'Widget',
				'desc'  => 'Do something with the Widget text.',
				'id'    => $prefix . 'region',
				'type'  => 'text'
			),
			array(
				'label' => 'Widget Paragraph',
				'desc'  => 'Tell us about the Widget in more detail.',
				'id'    => $prefix . 'paragraph',
				'type'  => 'textarea'
			),
			array(
				'label' => 'Widget Image',
				'desc'  => 'Add a portrait for the Widget.',
				'id'    => $prefix . 'image',
				'type'  => 'image'
			),
		);

		return $this->licensee_meta_fields;

	}

	/**
	 * Render widget fields
	 *
	 * Loop through the array of custom fields assigned in custom-field.php
	 *
	 * @since 1.0.0
	 * @return void
	 */
	public function render_widget_fields( $user ) {

		$custom_fields = $this->get_widgets_meta_fields();
		$post_type     = 'user';
		$id            = $user->ID;

		?>
		<input type="hidden" name="widget_meta_box_nonce" value="<?php echo wp_create_nonce( basename( __FILE__ ) ); ?>"/>
		<h3>Widget Fields</h3>
		<?php

		$this->render_custom_fields( $custom_fields, $post_type, $id );

	}

	/**
	 * Return active instance of Licensee Custom Fields, create one if it doesn't exist
	 *
	 * @since 1.0.0
	 * @return object Licensee_Custom_Fields
	 */
	public static function get_instance() {

		if ( empty( self::$instance ) ) {

			$class = __CLASS__;

			self::$instance = new $class;

		}

		return self::$instance;

	}

}
Widget_Custom_Fields::get_instance();

// js
https://gist.github.com/Magnacarter/68259b98590027bdb4d55596e8f2f53c

// custom fields library
https://gist.github.com/Magnacarter/ad8e2fa66fd2018017165ffe573a9067