Errors if "Default theme" is selected in the Administration Theme dropdown

Problem/Motivation

There are errors when viewing the Status Report if "Default theme" is selected in the Administration Theme dropdown at /admin/appearance:

Notice: Undefined offset: 0 in jquery_update_requirements() (line 67 of /var/www/html/sites/all/modules/contrib/jquery_update/jquery_update.install).

Notice: Trying to get property 'info' of non-object in jquery_update_requirements() (line 67 of /var/www/html/sites/all/modules/contrib/jquery_update/jquery_update.install).

Notice: Trying to access array offset on value of type null in jquery_update_requirements() (line 67 of /var/www/html/sites/all/modules/contrib/jquery_update/jquery_update.install).

Steps to reproduce

Use the same theme for site and admin, selecting "Default theme" in the dropdown.

The problem is with this code:

      $theme_key = variable_get($theme, FALSE);
      $theme_name = $themes[$theme_key]->info['name'];

$theme is 'admin_theme'. And running drush vget admin_theme in this case will return '0'.

Workaround

As a workaround, you can select the actual theme in the dropdown.

Proposed resolution

In this case, use the name of the default theme.