System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 445.87

Blender Version
Broken: version: 2.83.0, branch: master, commit date: 2020-06-03 14:38, hash: rB211b6c29f771
Worked: (newest version of Blender that worked as expected)

Short description of error
[Please fill out a short description of the error here]

Hi Blender team,

The Pose Library Thumbnails addon not installing in Blender official 2.83, tried a few times, with 2.82, 2.83, 2.90
and some of my friends cannot install it. Do not appear on the addon list.

addon by Sybren A. Stüvel : https://gitlab.com/blender-institute/pose-thumbnails
*I contacted him already and he suggested to do this bug report :)

@Hjalti Hjálmarsson (hjalti) used it for spring!!, I think this addon MUST be integrated in Blender official addons, is necessary for //character animation,//
(until you work on a new pose library in the future)

with a fix suggested by @umkastudio22
I `can install it, select the poses but is not showing the thumbnails (attached is a screen capture)
`
The fix:
Python API changed a lot in 2.80. You can fix addon by hand. Find http://prefs.py and change line 17 from

return context.user_preferences.addons[package].preferences

to

return context.preferences.addons[package].preferences"

must work"

Best Regards,
Leo

Exact steps for others to reproduce the error
[Please describe the exact steps needed to reproduce the issue]
[Based on the default startup or an attached .blend file (as simple as possible)]

The Pose Library Thumbnails addon not installing in Blender official 2.83, tried a few times, with 2.82, 2.83, 2.90
and some of my friends cannot install it. Do not appear on the addon list.