Fix: Exception: in Drupal\book\BookExport->bookExport()

modules/user_preferences/user_preferences.module

@@ -45,6 +45,7 @@ function user_preferences_node_links_alter(array &$links, NodeInterface $node, a
     $user = \Drupal::currentUser();
     $is_admin = in_array('administrator', $user->getRoles());
     $is_front = \Drupal::service('path.matcher')->isFrontPage();
+    $is_book = isset($node->book['bid']);
 
     if(!$is_admin){
         $can_add_child_page = in_array('Add content and child pages to books and manage their hierarchies', user_role_permissions($user->getRoles()));
@@ -122,7 +123,7 @@ function user_preferences_node_links_alter(array &$links, NodeInterface $node, a
         'aria-label' => 'Toggle fullscreen',
       ]
     ];
-    if($is_admin || $can_add_child_page){
+    if(($is_admin || $can_add_child_page) && $is_book){
         $link['addPage'] = [
         'title' => t('
             <span class="fa-layers">
@@ -155,7 +156,7 @@ function user_preferences_node_links_alter(array &$links, NodeInterface $node, a
         ];
         */
     }
-    if(!$is_front){
+    if(!$is_front && $is_book){
         $link['printerFriendly'] = [
         'title' => t('<i class="fal fa-print">&nbsp;</i>'),
         'url' => Url::fromUri('internal:/book/export/html/'.$nid),
@@ -164,7 +165,7 @@ function user_preferences_node_links_alter(array &$links, NodeInterface $node, a
             'data-toggle' => 'tooltip',
             'data-tippy-content' => t('Printer friendly version'),
             'aria-label' => 'Printer friendly version',
-        ]
+          ]
         ];
     }
     }