When converting an existing level with a large landscape (large enough to be spatially loaded), the resulting game world will crash in editor and build once landscapes parts are loaded spatially.

To fix this World Partition crash enable ‘Is Spatially Loaded’ on each landscape. Secondly, the landscape has to be set to loaded. ps. This was reported to the bug tracker today. [Landscape-World-Partition-Crash-Spatially-Loaded]