Deprecated warning about creating a dynamic property

Problem/Motivation

There are deprecated warnings about creating a dynamic property in the WidgetEntityAddController controller:

Deprecated function: Creation of dynamic property Drupal\stacks\Controller\WidgetEntityAddController::$storage is deprecated in Drupal\stacks\Controller\WidgetEntityAddController->__construct() (line 20 of modules/contrib/stacks/src/Controller/WidgetEntityAddController.php).

Deprecated function: Creation of dynamic property Drupal\stacks\Controller\WidgetEntityAddController::$typeStorage is deprecated in Drupal\stacks\Controller\WidgetEntityAddController->__construct() (line 21 of modules/contrib/stacks/src/Controller/WidgetEntityAddController.php).

I think this is caused by PHP 8.2+.

Steps to reproduce

1. Update to Drupal 10+ or PHP 8.2+
2. Navigate to /admin/structure/widget_entity/add

Proposed resolution

Declare the property in the class definition before the constructor, then assign the value in the constructor as before.