Redirect finance detail pages to their list when opened without an id

app/finance/manage_payment_billing.php

@@ -1,9 +1,17 @@
 <?php
   session_start();
   require '../config.php';
-  require '../include_header.php';
 
+  // A detail-only page: payment billings are created from their list, so with
+  // no id there is nothing to show. Send the user there instead of rendering an
+  // empty page that alerts "Payment billing id is required."
   $billing_id = (int)($_GET['id'] ?? 0);
+  if (!$billing_id) {
+    header('Location: ' . $server_url . 'finance/payment_billing.php');
+    exit;
+  }
+
+  require '../include_header.php';
 ?>
 
 <body>