finlabsindia/wp-content/plugins/ninja-forms/includes/Abstracts/SubmissionMetabox.php

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

abstract class NF_Abstracts_SubmissionMetabox extends NF_Abstracts_Metabox
{
    /**
     * @var array
     */
    protected $_post_types = array( 'nf_sub' );

    /**
     * @var NF_Database_Models_Submission
     */
    protected $sub;

    public function __construct()
    {
        parent::__construct();

        if( ! isset( $_GET[ 'post' ] ) ) return;

        $this->_title = __( 'Submission Metabox', 'ninja-forms' );

        $post_id = absint( $_GET[ 'post' ] );

        $this->sub = Ninja_Forms()->form()->get_sub( $post_id );
        
        add_action( 'add_meta_boxes', array( $this, 'add_meta_boxes' ) );
        add_action( 'save_post', array( $this, '_save_post' ) );
    }
}