WordPress Plugin - Notice: Undefined index error - php

Question

I'm playing around with a plugin that's a little out of date and trying to get it back up to standard but I've hit a brick wall. On line #25 of the php script below, it's throwing a Notice: Undefined index: zip_path error for if($entry['zip_path']){.
What's even more strange is that it's working on a couple of my domains but not on others. It might be worthwhile adding that the 'other' websites are hosted on external servers to the one I've been testing it on.
Can anyone provide some insight into why I'm receiving this Undefined index error?
<?php defined('ABSPATH') or die("No direct access allowed.");
require_once __DIR__ . '/vendor/autoload.php';
if (class_exists("GFForms") && !class_exists("GFBulkDownload")) {
class GFBulkDownload {
protected $name = 'Gravity Forms Bulk Download';
protected $version = '1.0';
public function __construct() {
add_action("admin_enqueue_scripts", array($this, 'addStyles'));
add_action("gform_entry_created", array($this, 'afterSubmission'), 1, 2);
add_action("gform_pre_submission_filter", array($this, 'preSubmissionFilter'), 10, 1);
add_action("gform_entries_column_filter", array($this, 'change_column_data'), 10, 5);
add_filter('gform_custom_merge_tags', array($this, 'custom_merge_tags'), 10, 4);
add_filter('gform_replace_merge_tags', array($this, 'replace_download_link'), 10, 7);
add_action("gform_entry_detail", array($this, 'addMetaToDetails'), 10, 2);
add_filter('gform_notification', array($this, 'before_email'), 10, 3);
}
function before_email( $notification, $form, $entry ) {
$site = trailingslashit(get_bloginfo('url'));
if($entry['zip_path']){
$url = $site.$entry['zip_path'];
$notification['message'] .= "\n\n <a href='{$url}' >Download All Files in Zip</a>";
}
return $notification;
}
public function addStyles(){
wp_register_style('gform_bulk_download_css', plugin_dir_url(__FILE__).'assets/css/style.css');
wp_enqueue_style('gform_bulk_download_css');
}
public function addScripts(){
}
function addMetaToDetails($form, $entry){
$zip = gform_get_meta($entry['id'], 'zip_path');
if($zip){
$site_url = trailingslashit(get_bloginfo('url'));
$url = $site_url.$zip;
echo "<table cellspacing='0' class='widefat fixed entry-detail-view'>
<tr>
<td colspan='2' class='entry-view-field-name' >Zip File Download Link</td>
</tr>
<tr>
<td colspan='2' class='entry-view-field-value lastrow'>
<a class='gfbd-btn' style='margin-left:0;' href='{$url}'> Download All Files Zip <i class='fa fa-download'> </i></a>
</td>
</tr>
</table>";
}
}
public function preSubmissionFilter($form) {
//Get upload path for form
$form_upload_dir = GFFormsModel::get_upload_path($form['id']);
//strip everything up to wp-content/.... so we can build a url
$form_upload_dir_formatted = stristr($form_upload_dir, 'wp-content/');
//decode json and remove slashes to get list of files
$files_for_zip = json_decode(stripslashes($_POST['gform_uploaded_files']), true);
if(!is_null($files_for_zip)){
//reset the array pointer
reset($files_for_zip);
// get the key of the first element (This is due to the format of the array )
$key = key($files_for_zip);
$zipArray = $files_for_zip[$key];
$zip = new ZipArchive();
$filename = 'files_' . md5(rand(0123456, 7890123)) . '.zip';
$file_path = trailingslashit($form_upload_dir) . $filename;
$zip->open($file_path, ZipArchive::CREATE);
foreach($zipArray as $file_info){
$zip->addFile($form_upload_dir . '/tmp/' . $file_info['temp_filename'], $file_info['uploaded_filename']);
}
$zip->close();
//Set the zip key for the $form object to the url for the zip file we created, minus the domainname so we can pass it to the entry
$form['zip'] = trailingslashit($form_upload_dir_formatted) . $filename;
}
return $form;
}
public function afterSubmission($entry, $form) {
//update the Entry with a zip_file meta field with a link to the zip file that was created.
if (isset($form['zip'])) {
gform_update_meta($entry['id'], 'zip_path', $form['zip']);
}
}
function change_column_data($value, $form_id, $field_id, $lead, $query_string) {
$form = GFFormsModel::get_form_meta($form_id);
$zip_path = gform_get_meta($lead['id'], 'zip_path');
$site_url = get_bloginfo('url');
$full_url = trailingslashit($site_url) . $zip_path;
foreach ($form['fields'] as $field) {
if ($field['type'] == 'fileupload' && $field['multipleFiles'] == "1") {
$upload_field_id = $field['id'];
}
}
if ($upload_field_id == $field_id) {
if (!empty($zip_path)) {
return $value . " <a class='gfbd-btn' href='$full_url'> Download All <i class='fa fa-download'> </i></a>";
}
}
return $value;
}
function custom_merge_tags($merge_tags, $form_id, $fields, $element_id) {
$merge_tags[] = array('label' => 'Download All Files (Zip)', 'tag' => '{download_all_files_zip}');
return $merge_tags;
}
function replace_download_link($text, $form, $entry, $url_encode, $esc_html, $nl2br, $format) {
$custom_merge_tag = '{download_all_files_zip}';
if (strpos($text, $custom_merge_tag) === false) {
return $text;
} else {
$zip_path = gform_get_meta($entry['id'], 'zip_path');
$full_url = trailingslashit(get_bloginfo('url')) . $zip_path;
return str_replace($custom_merge_tag, $full_url, $text);
}
}
}
new GFBulkDownload();
}
Thank you in advance.