If anyone is trying to play this through joiplay and gets the error "cannot read properties of null (reading 'videoWidth')", you can fix the error by deleting all of the xmp files in the following locations: "www/movies/", "www/sys/", and "www/kya/<enemyname>/".
As far as I know, xmp files are metadata files used by adobe software, so I don't think the game needs them to run.
If I had to guess, I think when joiplay looks for a video file, such as "a.webm", it might also find "a.webm.xmp" and try to use that instead. The error might come from trying to treat the xmp file as a video file.