I can create a material in Arch menu -> Material tools -> Material without any issues in this version:
OS: Windows 10 (10.0)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.17505 (Git)
Build type: Release
Branch: master
Hash: 755536e9df94d2d39da1468420f1fd333c35da7a
Python version: 3.6.8
Qt version: 5.12.1
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/United States (en_US)
But with this version FreeCAD exits with no explanation.
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16117 (Git)
Build type: Release
Branch: releases/FreeCAD-0-18
Hash: dbb4cc6415bac848a294f03b80f65e888d531742
Python version: 3.6.6
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)
It could be issue was fixed in 0.19 or perhaps one of the dependencies, such as Qt, had the issue and now has fixed it. In either case, I suggest trying the latest 0.19 build.
Not sure this will get a backport. Also, all the latest and greatest changes for Materials is on 0.19_pre so perhaps close this issue as 'Known Issue' ?
I am a new user and so perhaps not much saying about this. But I think a stable release should be as stable as possible. That is if you want to attract and hold new users.
Besides, what is otherwise the point of reporting a bug. I think that the solution for this problem is known, because in version 0.19 the bug is fixed.
I did try the pre-release of august 2. But simple models had problems with recomputing. Lines that I hide were shown after recomputing; object keep still touched after recompute. That can be expected with a development version, but it makes it not usable for a regular user at the moment.
I solved the problem with Arch Material in version 0.18.3 by copying the file "ArchMaterial.py" from the latest pre-release of version 0.19, https://github.com/FreeCAD/FreeCAD/releases/tag/Staging, to the folder ".\FreeCAD 0.18\Mod\Arch" of version 0.18 on my pc.