Forms Theme Editor Errors
I'm running Adobe Experience Manager 6.5.10.0 and getting the following errors in the console when I try to edit a Forms Theme.
It shows the loading graphic, downloads the jcr_content and throws the following errors.

I'm also seeing the following errors when trying to edit a form. The loading animation runs but the form editor never loads.

Then if I try to load a form in a page, I get "org.apache.sling.api.SlingException: Cannot get DefaultSlingScript". We were having issues on a previous service pack (6.5.7 and possibly 6.5.4) with the Forms Theme Editor just downloading the jcr_content, which isn't much of an issue in the long run and I worked around it, but now the forms won't load at all.
Is there anything I can do to fix this short of rolling back the SP?