I think the current behavior is not too bad. The Wikibase JavaScript code found some value it cannot allow you to edit because the UI is not able to represent the current state. What's broken about this is that the UI is not able to handle non-standard calendar models, but that's more of a feature request than a bug. We might also think about how to make the JS code more fail-safe, so that at least the remainder of it is executed when it encounters such an error, but I find that to be a minor problem.