WSOD error: The module thunder does not exist when updating from 2.5 to 2.6

I have a few sites which all use the thunder admin theme but do not use / have not been created using the thunder installation profile. I recently updated thunder admin to 2.6 from 2.5 and see a white screen of death (WSOD) when trying to access the status report and on inspecting recent log messages see the following:

Drupal\Core\Extension\Exception\UnknownExtensionException: The module thunder does not exist. in Drupal\Core\Extension\ExtensionList->get() (line 263 of /var/www/vhosts/example.local/httpdocs/core/lib/Drupal/Core/Extension/ExtensionList.php).

Reverting / downgrading do 2.5 fixed the issue.