Usually this error occurs a s a result of corruption in the Dreamweaver Site Definition. To fix this error, you will need to delete then recreate the site definition:
Go to Site manage Sites. Select the site you are trying to use the Web Standards Advisor on and click edit.
Go through each tab noting the settings that are used.
Once you have noted the settings, in the Manage Sites dialog, select the site and delete it, then recreate it again using the same settings.