It looks like You want to use export variables here.
What happens in your script is that best_file is nulkl at the start and it becomes string in process(). You didn't show it, but You call load function before process() can run, so an error occures, that File with path null can't be accessed.
when You introduce new variable You can do it like this :
export var newBest : String
Thanks to this, You can manually type desired path for each copy of this scene in editor, without making separate scripts for those scenes. One script, different variables. This way newBest will not be null on init() and You can refer to this variable anytime without null error.
Is this it, or did I take it too simply ?