Hey there! My apologies for the error - it looks like an engine update has changed how Ren'Py handles empty strings for displayables. To temporarily fix this, open the game and then open the console with shift+o and type in `persistent.sprt_bg = "black"` and hit Enter. Close the console and the tool should be possible to open. Clearing the background image will cause the error to appear again unfortunately, so I'll be putting out a fix for this as soon as I'm able. Thanks for the report!