That error means something else on your computer has the file open, and is able to write to the file. If you want to be extra sure that this error does not happen, then do this:
- Restart your computer. (Assuming whatever the process is doesn't load on startup, a reboot will shut down the mystery program).
- Before doing anything else, load up xDelta, and also open a folder containing the app.asar file.
- Use xDelta to apply the patch. Unlike the original instructions, call this patched file "app_patched.asar". Place app_patched.asar in the same place as app.asar.
- Close xDelta. Then, delete app.asar. If your computer won't let you delete app.asar, reboot again and delete the file.
- Then, rename "app_patched.asar" to app.asar.
- All should work fine at this point. You should be able to play.