Uncaught Error: Call to undefined function wp_get_current_user() in

You have to call TopAuthor() function in plugins_loaded orinit action. You are getting Fatal error: Uncaught Error: Call to undefined function wp_get_current_user() because wp_get_current_user() is a pluggable function. You can not call when your plugin is included. You have to wait for the action plugins_loaded or init. Call your TopAuthor() function using init or plugins_loaded action.

init action:

add_action('init','do_stuff');

plugins_loaded action:

add_action('plugins_loaded','do_stuff');

Your function :

function do_stuff()
{
    TopAuthor();
}

So full code of Your Plugin file look like:

<?php
/*
Plugin Name: Site Plugin for example.com
Description: Site specific code changes for example.com
*/
/* Start Adding Functions Below this Line */

/* Stop Adding Functions Below this Line */

// Register and load the widget
function wpb_load_widget() {
    register_widget( 'wpb_widget' );
}
add_action( 'widgets_init', 'wpb_load_widget' );

// Creating the widget
class wpb_widget extends WP_Widget {

    function __construct() {
        parent::__construct(

            // Base ID of your widget
            'wpb_widget',

            // Widget name will appear in UI
            __('WPBeginner Widget', 'wpb_widget_domain'),

            // Widget description
            array( 'description' => __( 'Sample widget based on WPBeginner Tutorial', 'wpb_widget_domain' ), )
        );
    }

    // Creating widget front-end
    public function widget( $args, $instance ) {
        $title = apply_filters( 'widget_title', $instance['title'] );

        // before and after widget arguments are defined by themes
        echo $args['before_widget'];
        if ( ! empty( $title ) )
            echo $args['before_title'] . $title . $args['after_title'];

        // This is where you run the code and display the output
        echo __( 'Hello, World!', 'wpb_widget_domain' );
        echo $args['after_widget'];
    }

    // Widget Backend
    public function form( $instance ) {
        if ( isset( $instance[ 'title' ] ) ) {
            $title = $instance[ 'title' ];
        }
        else {
            $title = __( 'New title', 'wpb_widget_domain' );
        }
        // Widget admin form
        ?>
        <p>
            <label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>" />
        </p>
        <?php
    }

    // Updating widget replacing old instances with new
    public function update( $new_instance, $old_instance ) {
        $instance = array();
        $instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
        return $instance;
    }
} // Class wpb_widget ends here


//add_action('init','do_stuff');
add_action('plugins_loaded','do_stuff');
function do_stuff()
{
    TopAuthor();
}

function TopAuthor() {
    $contributor_ids = get_users( array(
        'fields'  => 'ID',
        'orderby' => 'post_count',
        'order'   => 'DESC',
        'who'     => 'authors',
    ) );

    foreach ( $contributor_ids as $contributor_id ) :
        $post_count = count_user_posts( $contributor_id );
        // Move on if user has not published a post (yet).
        if ( ! $post_count ) {
            continue;
        }
        ?>
        <div class="contributor">
            <div class="contributor-info">
                <div class="contributor-avatar"><?php echo get_avatar( $contributor_id, 132 ); ?></div>
                <div class="contributor-summary">
                    <h2 class="contributor-name"><?php echo get_the_author_meta( 'display_name', $contributor_id ); ?></h2>
                    <p class="contributor-bio">
                        <?php echo get_the_author_meta( 'description', $contributor_id ); ?>
                    </p>
                    <a class="button contributor-posts-link" href="https://wordpress.stackexchange.com/questions/355429/<?php echo esc_url( get_author_posts_url( $contributor_id ) ); ?>">
                        <?php printf( _n( '%d Article', '%d Articles', $post_count, 'twentyfourteen' ), $post_count ); ?>
                    </a>
                </div><!-- .contributor-summary -->
            </div><!-- .contributor-info -->
        </div><!-- .contributor -->

    <?php
    endforeach;
}
?>

I have tested it runs without any error.

Activated plugin : https://prnt.sc/qhu87l
Registerd widget : https://prnt.sc/qhu8bj