I installed the shaper package and all updates on a current install of xammp with php 7.4.8.
I get this error:
Warning: Creating default object from empty value in D:\xampp\htdocs\j3\plugins\system\t4\themes\base\html\com_content\category\default_articles.php on line 22
From the category list at J! Pages > Category List
The page and its content otherwise seems to be fine (same as your demo), except the error message is at the top of the content. Every other page seems fine too, its only the category list.
Lines 21 and 22 from the referenced file are:
// Create some shortcuts.
$params = &$this->item->params;