Besides what Plokite_wolf said, while Gog does test their releases, the description shows how the bug wouldn't appear unless in a very specific situation. Imagine how many days of erratic actions in the game (and more precisely, in the game editor) would be needed until they reach the bug condition. Infinite monkeys with infinite t̶y̶p̶e̶w̶r̶i̶t̶e̶r̶s̶ computers, sure; but in the real world of limited resources it's no surprise it slipped past QA. Probably, as far as Gog knew .Net 3.5 isn't needed.

Glad the issue was solved tho. I suggest you post your finding in the game's subforum for future reference of people with the same issue.

Unfortunately It was just a fluke, It seemed to work briefly, but then started crashing again.
This is driving me nuts!

As I have 2 graphics cards, I changed over to the other card, again made sure any and all drivers are updated.
Still crashing.

If you are talking about why the scenario which you edited keeps crashing than it isn't fault of your computer or system. It is a fault of scenario which as I said before must have event, resources or other data connected to the coordinates. So you would have two options, either you will put the base of enemy in other place or look into the scenario(try by opening in the notepad) and look for the coodinates and check what is connected with it.

No, this is not the fault of 'the scenario'.
I've tested on the scenarios already built in the game, as well as my own previous created scenarios, as well as creating a new one. It is not scenario specific, it's editor specific.
This exact same game I've been playing for 3 years without a problem.
Suddenly this happened. I spent a day or two trying to figure out why it was happening, and other users testing my exact moves without a problem. Then my PC crashed, SSD drive was history. I assumed that whatever it was that made CotN crash in editor (on world level) was because of the faulty SSD drive.
Got the PC back with a new SSD drive, and a fresh install of Win 10. Installed my basic programs, installed Cotn, crashed.
Un- and re-installed many many times. I've changed any and all settings. Moved my monitor (only one now) to the nVidia card. Tried any and all install option there is (which I didn't need before).
Problem persist.
Makes no sense, but there you have it.