Fix profile.php errors.

public/profile.php

@@ -3,7 +3,14 @@ use Misuzu\Users\User;
 
 require_once __DIR__ . '/../misuzu.php';
 
-$user_id = (int)$_GET['u'];
+$user_id = (int)($_GET['u'] ?? 0);
+
+try {
+    $app->templating->vars(['profile' => User::findOrFail($user_id)]);
+} catch (Exception $ex) {
+    http_response_code(404);
+    echo $app->templating->render('user.notfound');
+    return;
+}
 
-$app->templating->vars(['profile' => User::findOrFail($user_id)]);
 echo $app->templating->render('user.view');

views/mio/user/notfound.twig

@@ -0,0 +1,15 @@
+{% extends '@mio/user/master.twig' %}
+{% from '@mio/macros.twig' import navigation %}
+
+{% set title = 'User not found!' %}
+
+{% block content %}
+    <div class="mio__container">
+        <div class="mio__container__title">User not found!</div>
+        <div class="mio__container__content">
+            <p>Check the url and try again.</p>
+        </div>
+    </div>
+
+    {{ navigation(mio_navigation, false) }}
+{% endblock %}