The error refers to google maps object. Are you using google maps on the page you are trying to open theme-it from?
We will try to look into this further, it appears to be a bug associated with running google maps but could occur with other scripts. It uses IE to create the preview. You could probably open up IE and turn of script errors and that could fix the problem in the meantime... or turn off preview and it should work without it until we can investigate it further... based on the error this appears to be a preview problem and not a problem with the actual functionality of the tool.
We will definitely look into this early next week to try to find a resolution. We can probably just remove the script from the page you are theming before creating the preview. Thank you for bringing this to our attention.