Add isset an multiple_dropdown_action (#1001)

* Add isset

* Update changelog

* Fix add to assign

Author: Lainow

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [UNRELEASE]
 
+- FIx undefined array key `multiple_dropdown_action` during import
 - Fix incompatibility of `multiple` dropdowns with `massiveaction`
 - Fix default value properly applied in multiple dropdown search options
 - Fix `search option` for default values in `multiple` dropdown

inc/container.class.php

@@ -1237,7 +1237,7 @@ public function updateFieldsValues($data, $itemtype, $massiveaction = false)
                 $field_name = 'plugin_fields_' . $field_data['name'] . 'dropdowns_id';
             }
             if (array_key_exists($field_name, $data)) {
-                if ($data['multiple_dropdown_action'] === 'append' && $exist) {
+                if (isset($data['multiple_dropdown_action']) && $data['multiple_dropdown_action'] === 'append' && $exist) {
                     // Add new values to existing ones
                     $existing_values = json_decode($obj->fields[$field_name] ?? '[]', true);
                     $new_values      = is_array($data[$field_name]) ? $data[$field_name] : [$data[$field_name]];
@@ -1831,7 +1831,7 @@ private static function populateData($c_id, CommonDBTM $item)
                 // ex my_dom[]
                 //in these conditions, the input is never sent by the browser
                 if ($field['multiple']) {
-                    $data['multiple_dropdown_action'] = $_POST['multiple_dropdown_action'];
+                    $data['multiple_dropdown_action'] = $_POST['multiple_dropdown_action'] ?? 'assign';
                     //handle multi dropdown field
                     if ($field['type'] == 'dropdown') {
                         $multiple_key         = sprintf('plugin_fields_%sdropdowns_id', $field['name']);