I have a problem with the T4_bs5_blank template and Joomla 4.2.1.
When I update my site tot Joomla 4.2.1 I get this error:
count(): Argument #1 ($value) must be of type Countable|array, null given
Only when I put the website off-line and logon on the frontend with my superuser account, the error does not appear.
For the moment, I still can use an older version of Joomla, but I will miss the security updates.