docs: add note for View Cells typed properties

Author: kenjis

user_guide_src/source/outgoing/view_cells.rst

@@ -101,6 +101,10 @@ Implementing the AlertMessage from above as a Controlled Cell would look like th
 
 .. literalinclude:: view_cells/010.php
 
+.. note:: If you use typed properties, you must set the initial values:
+
+    .. literalinclude:: view_cells/023.php
+
 .. _generating-cell-via-command:
 
 Generating Cell via Command

user_guide_src/source/outgoing/view_cells/023.php

@@ -0,0 +1,13 @@
+<?php
+
+// app/Cells/AlertMessageCell.php
+
+namespace App\Cells;
+
+use CodeIgniter\View\Cells\Cell;
+
+class AlertMessageCell extends Cell
+{
+    public string $type    = '';
+    public string $message = '';
+}