theme switcher only works when default permalink is selected, and then gives an error

I am creating a site for a blind charity and the users of the site will have different needs, such as yellow text on black, white text on black, black text on white, etc.

I thought the best solution was to use the Advanced Theme Switcher, but this is causing me problems.

It works fine when I have the default permalink structure, but as soon as I change the permalinks to something else (which some of my other plugins require), it doesn't change the theme.

When I do have the default permalink structure, it gives me the following error, which disappears if I select a different permalink structure:

Warning: array_key_exists() expects parameter 2 to be array, boolean given in C:\WampDeveloper\Websites\\webroot\wp-content\plugins\advanced-theme-switcher\advanced-theme-switcher.php on line 106.

I can't give you an example URL at this time, as I am designing it offline.