I don't quite have time to find the error, but I think it's because you've set the main script as a singleton and then run the main scene.
Create a script and call it
gloab.gd and put the score
var in that, along with any others you need. The load that script as a singleton, delete the
main_script.gd from the Autoload section.
That way, your script will load once.
Somewhere, the label is being set to a null, so when you try to set the text, it crashes.
If you can't find or anyone else doesn't reply, I'll take a look tomorrow.
It helped uploading the project I think.