An example "Resume" action for emulating the halt/redirect/resume process.

index.php

<?php if ( ! defined( 'ABSPATH' ) ) exit;

add_action( 'ninja_forms_register_actions', function( $actions ){
   include_once 'resume-action.php';
   $actions[ 'resume' ] = new Foo_ResumeAction();
   return $actions;
});

resume-action.php

<?php if ( ! defined( 'ABSPATH' ) ) exit;

/**
 * Class NF_Action_Custom
 */
final class Foo_ResumeAction extends NF_Abstracts_Action
{
    /**
     * @var string
     */
    protected $_name  = 'resume';

    /**
     * @var string
     */
    protected $_timing = 'late';

    /**
     * @var int
     */
    protected $_priority = 0;

    /**
     * Constructor
     */
    public function __construct()
    {
        parent::__construct();
        $this->_nicename = __( 'Resume', 'ninja-forms' );
    }

    /**
     * @param array $action_settings
     * @param int|string $form_id
     * @param array $data
     * @return array $data
     */
    public function process($action_settings, $form_id, $data )
    {
        if( ! isset( $data[ 'resume' ] ) ) {
            $data['halt'] = TRUE;
            $data['actions']['redirect'] = add_query_arg('nf_resume', $form_id, wp_get_referer());
        }
        return $data;
    }
}