[BUGFIX] Prevent PHP warning with missing items for a checkboxToggle

Resolves: #104834
Releases: main, 12.4
Change-Id: I0ec9d31b36e6fe885f020be895b5f6483c12d2b3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85900


Tested-by: core-ci <typo3@b13.com>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/backend/Classes/Form/Element/CheckboxToggleElement.php

@@ -73,7 +73,7 @@ class CheckboxToggleElement extends AbstractFormElement
             $disabled = true;
         }
         // Traversing the array of items
-        $items = $this->data['parameterArray']['fieldConf']['config']['items'];
+        $items = $this->data['parameterArray']['fieldConf']['config']['items'] ?? [];
 
         $numberOfItems = count($items);
         if ($numberOfItems === 0) {