How to solve PHP notices on plugin-install.php ?

I’ve suddenly started seeing these PHP notices at the top of my Add Plugins page…

Things I have done:

* Disabled all plugins.
* Changed theme.
* Deleted transients.
* Reinstalled 5.9.2.
* Logging-out and -in.
* Clearing out the plugins directory.
* Running the site with the [Safe Mode plugin](https://wordpress.org/plugins/safe-mode/) enabled (ie. virtually everything disabled).
* Removed browser data for this site.
* Run [wp_clean_plugins_cache();](https://developer.wordpress.org/reference/functions/wp_clean_plugins_cache/)

Nothing shifts it!

**What on earth has happened here, and how do I fix it?**

# 1:

    [01-Apr-2022 08:52:14 UTC] PHP Notice:  Undefined property: stdClass::$plugin in /Users/robert/Local Sites/the-map/app/public/wp-includes/class-wp-list-util.php on line 167
    [01-Apr-2022 08:52:14 UTC] PHP Stack trace:
    [01-Apr-2022 08:52:14 UTC] PHP   1. {main}() /Users/robert/Local Sites/the-map/app/public/wp-admin/plugin-install.php:0
    [01-Apr-2022 08:52:14 UTC] PHP   2. WP_Plugin_Install_List_Table->prepare_items() /Users/robert/Local Sites/the-map/app/public/wp-admin/plugin-install.php:41
    [01-Apr-2022 08:52:14 UTC] PHP   3. wp_list_pluck() /Users/robert/Local Sites/the-map/app/public/wp-admin/includes/class-wp-plugin-install-list-table.php:271
    [01-Apr-2022 08:52:14 UTC] PHP   4. WP_List_Util->pluck() /Users/robert/Local Sites/the-map/app/public/wp-includes/functions.php:5129

functions.php line 5129 relates to wp_list_pluck() … return $util->pluck( $field, $index_key );

# 2:

    [01-Apr-2022 08:52:14 UTC] PHP Notice:  Undefined property: stdClass::$plugin in /Users/robert/Local Sites/the-map/app/public/wp-includes/class-wp-list-util.php on line 167
    [01-Apr-2022 08:52:14 UTC] PHP Stack trace:
    [01-Apr-2022 08:52:14 UTC] PHP   1. {main}() /Users/robert/Local Sites/the-map/app/public/wp-admin/plugin-install.php:0
    [01-Apr-2022 08:52:14 UTC] PHP   2. WP_Plugin_Install_List_Table->prepare_items() /Users/robert/Local Sites/the-map/app/public/wp-admin/plugin-install.php:41
    [01-Apr-2022 08:52:14 UTC] PHP   3. wp_filter_object_list() /Users/robert/Local Sites/the-map/app/public/wp-admin/includes/class-wp-plugin-install-list-table.php:272
    [01-Apr-2022 08:52:14 UTC] PHP   4. WP_List_Util->pluck() /Users/robert/Local Sites/the-map/app/public/wp-includes/functions.php:5072

functions.php line 5072 relates to pluck() … $util->pluck( $field );

# 3:

    [01-Apr-2022 08:52:14 UTC] PHP Warning:  Cannot modify header information - headers already sent by (output started at /Users/robert/Local Sites/the-map/app/public/wp-includes/class-wp-list-util.php:167) in /Users/robert/Local Sites/the-map/app/public/wp-admin/admin-header.php on line 9
    [01-Apr-2022 08:52:14 UTC] PHP Stack trace:
    [01-Apr-2022 08:52:14 UTC] PHP   1. {main}() /Users/robert/Local Sites/the-map/app/public/wp-admin/plugin-install.php:0
    [01-Apr-2022 08:52:14 UTC] PHP   2. require_once() /Users/robert/Local Sites/the-map/app/public/wp-admin/plugin-install.php:136
    [01-Apr-2022 08:52:14 UTC] PHP   3. header() /Users/robert/Local Sites/the-map/app/public/wp-admin/admin-header.php:9

admin-header.php line 9 is: header( 'Content-Type: ' . get_option( 'html_type' ) . '; charset=' . get_option( 'blog_charset' ) );