null passed to hook_library_info_alter() after module installed causes fatal error

Does the error prevent installation of CSP?
Does the error occur again when clearing / rebuilding the cache?
Does the error occur when trying to install any other modules?

I wasn't able to reproduce hook_library_info_alter receiving NULL as the first parameter on a site with only csp.module (and not chosen).

I have the same issue.

When enabling / installing CSP module I am getting

he website encountered an unexpected error. Please try again later.

Drupal\Core\Asset\Exception\IncompleteLibraryDefinitionException: Incomplete library definition for definition 'php-markdown' in extension 'markdown' in Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension() (line 106 of core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php).

Site is not usable after this and I had to use drush to pmu csp.

1.10 is my version. But also happened in earlier versions.

I wasn't able to reproduce on a site installed with the Standard profile, CSP, and Chosen.

CSP doesn't make any alterations to library definitions, but it does scan for all available libraries, which could expose an underlying issue in the library definitions.
It's possible that it's a module other than CSP or Chosen that is causing invalid info in the library definitions - using a debugger to check the value of $module when hook_library_info_alter() is called with a null value for $libraries may uncover the source.

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The Markdown error looks likely to be a separate issue with Markdown 8.x-1.x and the contrib Libraries API module.

Same here after module install:

The website encountered an unexpected error. Please try again later.
TypeError: Argument 1 passed to locale_library_info_alter() must be of the type array, null given, called in /var/www/vhosts/thomas-zeithaml.de/httpdocs/core/lib/Drupal/Core/Extension/ModuleHandler.php on line 539 in locale_library_info_alter() (line 592 of core/modules/modules/locale/locale.module).

Cache cleaning has no effect.

Drupal 8.9.1
CSP Version: 8.x-1.12

Can anyone provide a more complete listing of the modules enabled on their site?

Most helpful of all would be steps to reproduce the error from an installation of the Minimal or Standard install profiles in core.

I experienced the same issue and found why it happened - there was an empty libraries.yml file in one of the custom modules.

I think the chosen module is wrong here because it assigns array type hint to hook_library_info_alter $libraries argument, which doesn't exist in original hook definition.

The hook docblock is wrong in that case too, it doesn't tell that it can be also null.

@rang501 Thanks for the info, I can also reproduce the issue now.

Core isn't consistent about the array type hint for hook_library_info_alter implementations, (only locale and responsive_image currently use it), but it shouldn't cause an error if used.

It looks like CSP exposes this issue because it needs to enumerate all libraries for enabled modules in order to build the default policy. Without CSP enabled, an empty library file would only be loaded if a (non-existent) library from that module was attached to the page.

I've created an upstream issue, and am working on a patch: #3167036: Empty *.libraries.yml file can cause fatal error

Core issue has been merged, so this will be resolved in the upcoming 8.9+ releases