@geoff777 Thank you for posting how you solved the problem! May I ask how does one upgrade themes and components? I have the same error but I only have two stock themes and no components at all. How do I install a Theme or Theme Component? does not describe any specifics of theme updates, only a way to install them from git. Did you have to overwrite the stock theme with the latest version from github?
2.6.0.beta4 to 2.7.0.beta1 in my case.
P.S. My guess is that you used https://github.com/discourse/discourse/blob/b5705348b32fff406d8e7e3463def65eaf635206/lib/tasks/themes.rake but I am not sure how to invoke it correctly to keep my install “upgradeable” (I don’t want to install any customizations).