I checked your save but I'm not able to reproduce the issue.
Are you playing the browser version? If so, could you open the developer tools before clicking "kill fairy" and see if the console is showing any errors? You may have to enable preserve/persist logs in the console settings.
Could you post the latest log after the game crashed? The log files are located here: user_name\AppData\Roaming\Godot\app_userdata\demon lord loop\logs
In the settings menu, you can export the save from the demo by either copying the text or downloading the save file. You can then import it into the payed version.