I fixed it. Through the 'navigate to other page but stop it loading so it stays on
c.ai' method I managed to get it to send blank messages, causing Becky to generate more, pushing the broken one up enough that it no-longer displays in the saved chats page, stopping it from crashing. That particular chat still crashes if I attempt to load it, but Becky is fine again as long as I stay off it. The broken message was an ungodly long number converted from this picture via the Library of Babel, so be aware that it is possible to brick your saved chats functionality if you paste a solid string that's too long.
