Too few arguments to function WP_Widget::__construct(),

You’re using an extremely outdated method of creating a widget. You should be using the __construct() function, not a named function, as the constructor, as documented.

namespace App;

class My_Widget extends \WP_Widget {
    function __construct() {

        $widget_ops = [
            'name' => 'My widget',
            'description' => 'My description'
        ];
        parent::__construct( 'My_Widget', 'My widget', $widget_ops );
    }
}

function load_widget() {
    register_widget( 'App\\my_widget' );
}
add_action( 'widgets_init', __NAMESPACE__ . '\\load_widget' );