This. This error usually means the JavaScript engine crashed, and the typical source is skins and mods. It happens on desktops, too. If you're using any that are resource intensive, it'll cause this. Downloading a game increases the likelihood of this, because the Steam downloader requires a lot of CPU power.What were you doing when that happened (like does it occur when you open a certain menu, or do a specific thing, or just how do you replicate it), do you have decky-loader installed, and if you do, what plugins do you have?
Did you update recently? Are you on the stable branch? or are you on preview?