That is quite normal.
That script requires you to start a new game. After adding that script, old save files will NOT work anymore.
As a general rule, you should always start a new game when you add new scripts to your project.
And this is why you are better off implementing any scripts you might need at the very start of your game development.
Adding new scripts in the middle of your development progress might get you into some complications later on (such as balancing issues, not working save files, etc).