[BUGFIX] Return buttons if page is not callable

The ButtonBarHook seems to be called on root (pid=0) or other possible
constellations, too. In these cases (and some other) the BackendUtility
can't return avalid page record and returns null instead. Following up
code didn't check, if a valid page record was loaded from the database,
which lead to errors. WIth this commit, a check on a `$page === null`
and hardening on possible not set array keys is added avoiding this issue

Classes/Hooks/ButtonBarHook.php

@@ -34,16 +34,17 @@ public function getButtons(array $params, ButtonBar $buttonBar): array
             return $buttons;
         }
 
-        /** @var array{uid: int, doktype: int, module: string} $page */
+        /** @var array{uid: int, doktype?: int, module?: string}|null $page */
         $page = BackendUtility::getRecord(
             'pages',
             $queryParams['id'],
             'uid,module,doktype'
         );
 
         if (
-            (int)$page['doktype'] !== PageRepository::DOKTYPE_SYSFOLDER
-            || $page['module'] !== 'glossary'
+            $page === null
+            || (int)($page['doktype'] ?? 0) !== PageRepository::DOKTYPE_SYSFOLDER
+            || ($page['module'] ?? '') !== 'glossary'
         ) {
             return $buttons;
         }