I have seen this failing after updating to beta12 as well.
It seems like the URL format matters.
https://gitlab.com/silentboatman/discourse-masonry-image-gallery/ → error
https://gitlab.com/silentboatman/discourse-masonry-image-gallery → error
https://gitlab.com/silentboatman/discourse-masonry-image-gallery.git → works
That means that you will need to remove and re-add theme components that have one of the first two formats, and copy the configuration. But it would be pretty cool if those two formats would be supported out of the box as well.