This crash seems to only be reproducible using the TallGrass_01 asset in QAGame. Having other foliage assets selected sometimes prevents the crash, but usually any combination of assets plus TallGrass_01 will crash the editor when attempting to paint foliage onto a landscape.
This is a regression and does not happen in 4.16.3
Preparing to test in Main.
Result: Editor crashes immediately.
Expected Result: Editor paints the selected grass asset cleanly onto the landscape.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-48035 in the post.