When adding a sub-level that contains a Landscape and Landscape Splines to an existing Level, it will cause a crash when building the lighting.
This issue was reported in 4.18.3 (CL 3832480) and tested in 4.19.2 (CL 4033788), 4.20 Preview 3 (CL 4135216), and Main 4.21 (CL 4151283)
Results: The Editor crashes
Expected: Lighting builds and the Editor doesn't crash
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60909 in the post.