Plesk is unable to install or update WordPress instances with WordPress Toolkit: Db::makeUniqueDbName() must be of the type int, null given

Symptoms

• Installing WordPress on a domain fails with the following message:

  PLESK_WARN: Your hosting package does not provide for the creation of additional databases. This WordPress installation uses an existing database instead of a new one.

• Errors like the following can be found in /var/log/plesk/panel.log:

  CONFIG_TEXT: [2023-01-01 01:23:45.678] ERR [panel] Task failed: id=12345, pid=9480, type=ext-wp-toolkit-smart-update-instance, error=Argument 1 passed to PleskExt\WpToolkit\Helper\Db::makeUniqueDbName() must be of the type int, null given, called in /opt/psa/admin/plib/modules/wp-toolkit/library/Servers/DomainDatabaseManager/PleskDomainDatabaseManager.php on line 76
  [2023-01-01 12:34:56.789] ERR [extension/wp-toolkit] Internal Server Error: Argument 1 passed to PleskExt\WpToolkit\Helper\Db::makeUniqueDbName() must be of the type int, null given, called in /opt/psa/admin/plib/modules/wp-toolkit/library/Helper/Install/PleskInstallTargetDatabase.php on line 144

• There are multiple MySQL instances in Tools & Settings > Database servers

Cause

WordPress Toolkit bug with ID EXTWPTOOLK-10137

Resolution