Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: Intel(R) HD Graphics 4600 Intel 4.3.0 - Build 18.104.22.16835
Broken: version: 2.80 (sub 54), branch: master, commit date: 2019-04-05 23:55, hash: rBf12040e088b6
Short description of error
The python function - bpy.data.libraries['yourlib'].users_id() function throws an error, becase it looks for 'grease_pencil' in the function, while it should be "grease_pencils" in 2.8
can be fixed in /scripts/modules/bpy_types.py on line 64.
(I don't have commit rights)