This is due the fact that the Registry class doesn't allow null values anymore. More information about this topic can be found in #20675. The consequence is that every instantiate or merge needs to be surrounded by an if block. So $registry = new Registry($value); has to become:

if ($value)
{
    $registry = new Registry($value);
}

@laoneo It may be another discussion but perhaps the Registry should return an empty Registry object?

@mbabker is here the man in charge. I really don't care at all as long as we make a proper announcement when we are more strict in J4 than J3, as this class is used widely in core and 3rd party extensions.

So $registry = new Registry($value); has to become:

No, constructor still allow you to put empty string or null.
Michael only want to change loadString() method.

On J4, you can use:

$registry = new Registry('');
$registry = new Registry(null);

but you can not:

$registry = (new Registry())->loadString('');
$registry = (new Registry())->loadString(null);

Take a look at the constructor at https://github.com/joomla/joomla-cms/blob/4.0-dev/libraries/vendor/joomla/registry/src/Registry.php#L65

Currently, Registry has been updated for J3.x and it is not strict, J4.x has an older version with strict code.

Ok then I was wrong. Not sure if Michael is pushing that change into 2.x.

For me 2.x should be strict.

As I said. I'm fine with both options. Np.

The CMS is behind a bit on some of the Composer dependencies in 4.0. As far as the framework repos go everything committed to master (1.x) is being merged regularly into the 2.0-dev branches, so there are few if any behavioral differences aside from intended changes. We just need to composer update things here, a lot of packages haven't been updated in a while.

Small reminder: we also have the fatal error concerning the class declaration here, and same for Newsfeeds. ?

has this been resolved?

Did #20802 fix this issue?

Closing as this has indeed been solved.