Monster Builder crashes when trying to add Class Graft

Selecting a Class Graft causes the screen to clear, showing only the background.

Refreshing the page brings up:

Page not found
Looks like you’ve followed a broken link or entered a URL that doesn’t exist on this site.

If this is your site, and you weren’t expecting a 404 for this path, please visit Netlify’s [“page not found” support guide](https://answers.netlify.com/t/support-guide-i-ve-deployed-my-site-but-i-still-see-page-not-found/125?utm_source=404page&utm_campaign=community_tracking) for troubleshooting tips.

Reproduce the issue in Monster Builder by selecting a Class Graft, then refresh the page after the screen clears to confirm the resulting 404. Trace the navigation or rendering path triggered by that selection; done means the Class Graft screen remains visible and refreshing it no longer shows a page-not-found error.

Written by the indexing model from the issue text.