I have this problem as well and found some ways to test for it. First, hide everything in your scene with the visibility icon in the world outliner tab. Then switch to lightmap density viewmode. If it crashes here you might have something wrong with your ue4 install. But for me, as I turned on the visibility to each object in the scene, it would always crash on the same object. Upon closer inspection it was the material that was crashing the viewmode. The material had flat tesselation (Crack free) turned on. Once I turned it off, I would turn on the visibility of the object in lightmap density viewmode and it would work fine. Note that it may not be tesselation causing the problem in your material (it was for my case). But it most likely is the materials fault for the crash. To test that just apply a default material and turn on the visibility of the object to test for the crash. If it doesn’t crash with the default material it is definitely the materials fault for the crash.