Reproduction steps:

1. Create a New Light Weight RP Project
2. Upgrade the LWRP through the package manager to '4.1.0-preview'
3. Select "Main Camera" -> "Viewport Rect" and change "H" or "W" values to anything

Expected Result: The values change
Actual Result: The Editor freezes

Reproduced with: 2018.3.0b10, 2018.3.0b1 ("LWRP v4.1.0-preview only works on 2018.3.x")
Did not reproduce on: 2019.1.0a9, 2018.3.0a11, 2018.2.16f1, 2018.1.9f2, 2017.4.15f1

Note:
Only reproduces with LWRP v4.1.0
Can reproduce on an empty scene
Sometimes you have to change the values twice (Or just both of them)
Only reproduces with MacOS 10.14