Indeed. I have been pretty heavy into property graph schemas for a while now, and I don't know of a good UI for editing them. There are plenty of tools you can use for visualizing a schema, but none that are particularly good for authoring. If there were, I would be using them, and trying to get others to use them at Uber.
Gra.fo is possibly the next best thing to a property graph schema editor right now. Conversations have been had about adding support for APG-style schemas to Gra.fo, but it doesn't look like this will happen soon.
That said, if you don't actually need a UI, I am preparing to release a tool called Dragon which will help you work with graph schemas, validate them, combine them, and transform them to/from a variety of formats.
Josh