This happened to me in 4.8 and 4.9.
If you create a hole in the landscape with one material, and then you decide you want to clear the material or change to a different one, the editor will crash and it is impossible to change material unless you first remove all holes in the landscape.
From what you have described you created a landscape and added a material for it and for a hole. Then you make a hole using the visibility tool. Then you cleared either the hole material or the landscape material. I have tested for both to make sure. After you do that the editor does crash.
Using what I just described I tested this on my machine and was unable to reproduce the crash.
If you would link me your DxDaig and then tell me if any of the steps I used differ from yours. I will then see if I am able to test this further.
Steps to reproduce the crash on my end.
Create hole with visibility tool (not using the old visibility material method)
Try to clear or change the landscape material… Immediate crash.
I’ve made a video which you can see here.
Though for some reason the dropdown menus don’t show up using xsplit to record… But as you can see… I use the most simple material possible with just a color and landscape visibility mask
Then I create a hole, and then try to clear the material and it crashes.
Here is my DxDiag
There have been multiple reports for Radeon Series 200 graphics cards. We are currently looking into these issues.
I have entered a bug for you. The number is UE-20708.
Our development team will look into this as soon as they are able.