bedna - yes, that’s the original which they say is causing the problem and you have to delete lines 141-161 and replace the nested if statements with the code in the earlier post.
In my case, the problem is that when I overwrite the original, it doesn’t seem to be used by calamares so I guess there’s something else to be done.
Anyway, hopefully it’s fixed in the next release - as you say.
I have the same error in neon-user-20240303-0009. But with this solution I was not able to fix this.
Here I have once again uploaded the “main.py” file that works for me.
I delete the original file and add the new one.
Maybe there is a problem with overwriting.
I hope that the download now works without any problems.
https://www.swisstransfer.com/d/afe0759f-39f6-4620-95e7-d9e87a5c3d19
I just posted the original in case someone lost it in the process of trying to modify installation configuration files…
I am NOT going to spoonfeed anything here, but want to give you guys a friendly warning.
Do you guys have any idea what editing that file does with your installation?
(I don’t, I did not analyze any code).
If you can not edit a file on a usb stick without help, maybe you should not do things like this.
If you have to download a file someone else has uploaded because you do not know how to edit a file, maybe not ok.
Just a thought and a friendly heads up.
1- start the live cd
2- download the main.py file from this link
https://www.swisstransfer.com/d/afe0759f-39f6-4620-95e7-d9e87a5c3d19
3 - open the terminal and run dolphin with this command
sudo pkexec env DISPLAY=$DISPLAY XAUTHORITY=$XAUTHORITY KDE_SESSION_VERSION=5 KDE_FULL_SESSION=true dbus-launch dolphin
4 - Navigate to the folder
/usr/lib/x86_64-linux-gnu/calamares/modules/networkcfg/
and delete the main.py file and paste the main.py file that you downloaded at the place
5 - Log out of the account
6 - disconnect from the internet
7 - install your kde neon Good Luck 
??
Ok…
Last warning, unless someone from KDE comments with “this is completely fine” I strongly advice against this.
This worked! thank you so much
This fix did work on a fresh install from the 20240303-0009 iso. Editing the existing file for the fixed entry didn’t work, but replacing the file did. You also don’t log out, you close the dolphin window after replacing the file and start the installer.
There is probably an update for calamares, among other things.
If the update is carried out in the live version before installation, an error message no longer appears.
A new ISO has just been released.
This should at least solve this problem. 
The error has been fixed with neon-user-20240304-2101. I was able to test it today. Thank you very much.