Updated Sept.9.2018The post linked below gives a tip on how to stop the script error from happening.
Sims 3 Launcher Running in Tray While Playing
https://answers.ea.com/t5/The-Sims-3/Sims-3-launcher-running-in-tray-while-playing/m-p/7010161#M231841The script error doesn't effect gameplay, meaning you can still play the game. It's just that if you open the launcher at any point during the game, you will need to manually close it down or use Task Manager to close it. Open Task manager (Ctrl-Shift-Esc) search for a file called Sims3Launcher (32 bit) Right click on it and select End Task.
To fix this, see the tip Puzzlezaddict gives. I've tried it and it works really well and I can still add files to the downloads folder and install files.
I've seen a similar topic here recently. Here's the link:
Script Error when opening launcher
http://www.carls-sims-3-guide.com/forum/index.php/topic,11569.0.html---
Yes, it's an error. But it's not a user created error. It's something EA will need to fix, as it happened with one of the recent update patches.
Here's a quote, from Flynn, of what's happening - why the script pops up.
The error is in the html page the launcher is trying to display. Clicking Yes to continue running scripts is fine and won't hurt anything. EA will likely fix it in an upcoming site update.
Flynn
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Although, recently, within the last couple of weeks, I noticed that I don't have the script error pop up. But I've been playing my game saves in a new Sims 3 folder, without store content.