If notifications.email is a string, an error occurs

Thank you for the patch and for the stack trace.

This seems to be an unusual situation. I don't understand how $notification_emails could be unset at this line because just above this section it is set as:

$notification_emails = $config->get('notification.emails');

I think $notification_emails could be null here, but only if there is something wrong with the site configuration. notification.emails is set to an empty array on install and it is part of the update module schema. What does a var_dump of $notification_emails show?

I think the steps to reproduce need some refinement.

I was having the same issue. Uninstalling and then reinstalling Update Manager fixed it. The site is using Drupal 9.5.8 and PHP 8.1.13.

I am having the same issue.
$notification_emails is NULL.

The patch looks good and makes sense to include it in the core, IMO.
Calling a function that can break the page on unchecked variables is much worse than just adding a normal safety check. Perhaps the missing values come from old installs/configurations and/or migrations when it wasn't necessary. Reinstalling + importing configs will still remove the initially installed empty array when one wants to keep the original settings.

You can also just manually add the missing value in update.settings.yml and import configs to resolve the issue as well, but better safe than sorry?

notification:
  emails: {  }

The suggested patch (explained in Proposed Resolution) worked for me, too. This D9 site on php 7.4 is being run on php 8.1.22 (8.0.0 removed the legacy param order, per changelog on php.net; info thanks to this smart guy)

I found a way to reproduce this which I added to the IS.

Turns out that notification.emails can be saved as string, an array or NULL. And it doesn't validate the email address either.

I wonder if there are duplicates of this.

Is this a problem in D10.2 or D11?

#9 seems @quietone was able to reproduce in 11.x branch.

Still a problem in D10.2, attached patch with 10.2 version compatibility.

The Needs Review Queue Bot → tested this issue.

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Raised MR !6804 of patch #13.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

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Fixed core commit check issues. Changing status to Need Review.

Will need test coverage since it caused a fatal error.