Hi, I had the same initial error as bcnx and located the problem. Upgraded from Joomla 3 to 4 and despite having prepared, updated the template and extensions, had the error CANNOT ACCESS PROTECTED PROPERTY JOOMLA\CMS\MENU\MENUITEM::$PARAMS.
I experimented with changing to Cassiopeia template, unpublishing all modules and turning them on one by one. I discovered the error came from a menu module. In the J3 template there was an override of the mod_menu (templates/gk_steakhouse/html/mod_menu) with file default.php, but this is not in the updated/J4 template.
Since the new version doesn't include that file, the override isn't changed or removed, and remains to cause the error. If you delete the file the site should work.