Operating system: Linux-4.15.0-58-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.40
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
2.79 as well
Short description of error
Just noticed this behavior. If a mesh doesn't have a vertex group with at least one vertex in it, then the object disappears with the mask modifier in use. Vertex group option in the modifier is empty.
I don't know if it's a bug or it is by design. But anyway, this is not very intuitive.
Exact steps for others to reproduce the error
Create an object
Assign the mask modifier
Create a vertex group with at least one vertex and the object comes back