How to reproduce:

1. Open any (new) project
2. Import attached script
3. Attach the script to any gameobject
4. Set the World Extents X value to zero
- Note there is no error
- Setting it to a negative value produces an error which says that the "world bounds have to be non-empty"
- This does not seem to affect the simulation; if one of the axes is set to 0 but but any of the simulated objects intersect the remaining plane, they will correctly generate contacts

Reproduced with: 2018.3.0a3 (0ac1bca97520)