Yeah, I need you to copy that script and give it to me. I finally got the demo downloaded, and line 788 in that script is a blank line. So you've made changes to it, and the demo script isn't much good to help figure it out, as I can't tell which line has the error.
Hold on ... I don't think it's got anything to do with a script incompatibility. You said you go to load a save file ... are these several save files that are on the maps with and without fog, and it's the maps with fog that have the problem? Any chance those save files were created BEFORE you added the weather effects script, or the games were started before the script was added?
The only reference to .empty? in that script is to do with some variables set up in Game_System's initialize method. Which means if you're loading save files created before you added the script, they are going to crash. The fog script does nothing with Game_System.
Try starting a NEW game with the starting point situated on each of those maps in turn, and see if the problem still happens.
undefined method ... for nil:NilClass
THIS error is ALWAYS a big clue that you might be playing an old save, and new scripts have been added. That error would DEFINITELY happen with the weather effects script, when you try to resume a game that was in progress and had been saved prior to adding the script.