map editor is crashing much more in 14 than it did in 13

We know.
The editor is being taken apart to be fixed properly, but this needs time and you won't see that until it's ready to be shown.

Meanwhile, any steps on how we can replicate this crashing issue?

 

I had a look at the map, and I see that it spams lot of errors because of the groundmodels:

It causes the editor to lag when you are painting/modifying the terrain.
I guess that after 15 minutes of continues lag and error spam, these will start to be heavy on the memory.
I'd suggest fixing those errors meanwhile, should be as simple as assigning correct/existing groundmodels to those terrain materials.


It indeed crashed the game here as soon I edited the terrain too.

 

Uhm, having a second look again, I'm not sure why that error is showing up at all.
Your materials.cs file in the terrain folder (where the terrain mats are) seems fine, although it does not seem to load some of them.
From my own experience that can happen if there's a comma or bracket missing, making the game load the materials up to that point, but unable to load the ones after the error in the .cs file.
Although your .cs file looks fine to me.

Need a bit of time so we can get to check the crash (weekend).

Also, there are also some errors about missing textures.
From experience again, these can cause crashes too.
These should be easy to fix, so I'd start with those meanwhile.