I found out you can delete all the "textures" - except the last one. If you want to remove all from the terrain editor list - then it crashes.
So in case none of the above works delete one by one from the terrain editor list, until you have one to go, apply some new texture or just fill something new into the terrain editor list - and then delete the previous last one.
That read writing into protected memory got me checking and rechecking my partition security options many times, but of course its not that.
Also, guise... maybe we are not meant to paint the terrain from the terrain editor?
Maybe we should paint from the Instances painter?
Could be, you never know....