This bug has been around for the last few engine versions, but I realized switching to lightmap density view mode crashes every time I run my project with DX12 RHI. Switching back to DX11 seems to resolve the issue. I think these are the steps to reproduce:
1 - Create new project
2 - Switch to DX12 RHI
3 - Restart and reopen project
4 - Switch to lightmap density view mode
Lightmap density view mode should display, but UE4 crashes instead.