Below the My_Plugin_Widget class, add a function named my_plugin_register_widgets that registers the widget.

<?php
class My_Plugin_Widget extends WP_Widget {

    function my_plugin_widget() {
        parent::__construct( false, 'My Plugin Widget' );
    }

function widget( $args, $instance ) {
extract( $args );
  $title = apply_filters( 'widget_title', $instance['title']);
}
function update( $new_instance, $old_instance ) {
$instance = $old_instance;
  $instance['title'] = strip_tags($new_instance['title']);
  return $instance;
}
function form ( $instance) {
$title = esc_attr($instance['title']); 
  require ( 'inc/widget-fields.php'); 
}
    function my_plugin_register_widgets() {
        register_widget( 'My_Plugin_Widget' );
    }
    add_action('widgets_init', 'my_plugin_register_widgets' );
}

?>