fix: add default value for dropdown to avoid empty dropdown (#915)

* fix: add default value for dropdown to avoid empty dropdown

* Update CHANGELOG.md

* fix multiple input error

* update CHANGELOG.md

Author: MyvTsv

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## [UNRELAESE]
 
 - Fix container update from `API`
+- Fix: fix default value for `dropdown` field to avoid empty dropdown
 
 ## [1.21.18] - 2024-01-16
 

ajax/field_specific_fields.php

@@ -119,11 +119,17 @@ function ($itemtype) {
         if ($field->isNewItem() && $type == 'dropdown') {
             echo '<em class="form-control-plaintext">';
             echo __s('Default value will be configurable once field will be created.', 'fields');
+            if (!$multiple) {
+                echo '<input type="hidden" name="default_value" value="" />';
+            }
             echo '</em>';
         } else {
             $itemtype = $type == 'dropdown'
                 ? PluginFieldsDropdown::getClassname($field->fields['name'])
                 : $dropdown_matches['class'];
+            if ($field->fields['default_value'] === null) {
+                $field->fields['default_value'] = '';
+            }
             $default_value = $multiple ? json_decode($field->fields['default_value']) : $field->fields['default_value'];
             Dropdown::show(
                 $itemtype,