admin-ajax 400 Bad Request

Question

Hi I have a plugin class which I am modifying.

I am trying to create an ajax call to call a function from php on button click. This is my code:

function import() {
    $this->get_file();
    echo ("<h1>Sample of the first 3 records in file</h1><br>");
    $this->get_sample_of_records();
    echo("<button type=\"button\" onclick=\"import_file()\"> Import </button>");        
}

In Javascript this is my jquery:

    function import_file () {
     jQuery.ajax({
        url: ajaxurl, 
        data: {'action': 'post_final_save'},
        method : 'POST', //Post method
        success:function(result){
         alert(result);
       }
     });
}

This is the function I want to call:

public function final_save(){       
    echo 'here';
    die();
}

And this is where the class is initialized and the action hook called:

 function really_simple_csv_importer() {
    load_plugin_textdomain( 'really-simple-csv-importer', false, dirname( plugin_basename(__FILE__) ) . '/languages' );
    
    $rs_csv_importer = new RS_CSV_Importer();
    add_action( 'wp_ajax_post_final_save', array( $rs_csv_importer, 'final_save' ) ); 
    add_action( 'wp_ajax_nopriv_post_final_save', array( $rs_csv_importer, 'final_save' ) ); 
    register_importer('csv', __('CSV', 'really-simple-csv-importer'), __('Import posts, categories, tags, custom fields from simple csv file.', 'really-simple-csv-importer'), array ($rs_csv_importer, 'dispatch'));
}
add_action( 'plugins_loaded', 'really_simple_csv_importer' );

This is the dispatch function called by register_importer:

function dispatch() {
    $this->header();
    $this->enqueue_script();


    if (empty ($_GET['step']))
        $step = 0;
    else
        $step = (int) $_GET['step'];

    switch ($step) {
        case 0 :
            $this->greet();
            break;
        case 1 :
            check_admin_referer('import-upload');
            set_time_limit(0);
            $this->import();                
            break;
    }
    
    $this->footer();
}

I am getting the following error:

/wp-admin/admin-ajax.php 400 (Bad Request)

Any idea why and how I can fix it please?

To add a little more context - This class is extending WP_Importer

if ( !defined('WP_LOAD_IMPORTERS') )
    return;

// Load Importer API
require_once ABSPATH . 'wp-admin/includes/import.php';

if ( !class_exists( 'WP_Importer' ) ) {
    $class_wp_importer = ABSPATH . 'wp-admin/includes/class-wp-importer.php';
    if ( file_exists( $class_wp_importer ) )
        require_once $class_wp_importer;
}

// Load Helpers
require dirname( __FILE__ ) . '/class-rs_csv_helper.php';
require dirname( __FILE__ ) . '/class-rscsv_import_post_helper.php';



/**
 * CSV Importer
 *
 * @package WordPress
 * @subpackage Importer
 */
if ( class_exists( 'WP_Importer' ) ) {

class RS_CSV_Importer extends WP_Importer {