added check if user is in route

config/install/views.view.user_personal_notes.yml

@@ -200,7 +200,7 @@ display:
           group_type: group
           admin_label: ''
           plugin_id: text_custom
-          empty: false
+          empty: true
           content: '<a href="/user/{{ arguments.user }}/add-note" class="button button--action button--primary">Add Note</a>'
           tokenize: true
       footer: { }

src/Plugin/Block/PersonalNotesBlock.php

@@ -8,6 +8,7 @@ use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Session\AccountProxyInterface;
 use Drupal\personal_notes\Entity\PersonalNote;
+use Drupal\user\UserInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -79,7 +80,7 @@ class PersonalNotesBlock extends BlockBase implements ContainerFactoryPluginInte
    */
   public function build(): array {
     // User must be logged on to have personal notes.
-    if (!$this->currentUser->isAnonymous()) {
+    if (!$this->currentUser->isAnonymous() && $this->routeMatch->getParameter('user') instanceof UserInterface) {
 
       $user = $this->routeMatch->getParameter('user');