fix: guard against null contentElement in renderer, clean up LANG global in tearDown

Author: konradmichalik

Resources/Public/JavaScript/frontend_edit.js

@@ -819,7 +819,7 @@
       let failed = 0;
 
       for (let [uid, contentElement] of Object.entries(jsonResponse)) {
-        if (!contentElement.menu || !contentElement.element) {
+        if (!contentElement || !contentElement.menu || !contentElement.element) {
           Logger.log(`Skipping content element c${uid}: missing menu or element data`, null, 'warn');
           failed++;
           continue;

Tests/Unit/Service/Menu/ContentElementButtonBuilderTest.php

@@ -53,6 +53,7 @@ protected function setUp(): void
     protected function tearDown(): void
     {
         GeneralUtility::purgeInstances();
+        unset($GLOBALS['LANG']);
     }
 
     #[Test]