In 261320@main I forgot to update the marshaller used by WebKitUserContentManager::script-message-received. It worked perfectly fine in my development environment for whatever reason, but was broken in at least Ephy Tech Preview.
Um, this is a speculative fix because I can't reproduce the crashes in my development environment, but it's correct and I'd be surprised if this isn't enough.
Pull request: https://github.com/WebKit/WebKit/pull/11664
Committed 261810@main (fa61ab3f2467): <https://commits.webkit.org/261810@main> Reviewed commits have been landed. Closing PR #11664 and removing active labels.