System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.87

Blender Version
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-02-12 16:20, hash: rB77d23b0bd76f
Worked: (optional)

Short description of error
If a blender file saved previously with the render engine set to bpy.context.scene.render.engine = 'BLENDER_WORKBENCH' is opened again
Many addons that rely on previews uploaded via bpy.utils.preview.new (), like 'ui_previews_dynamic_enum.py' or 'ui_previews_custom_icon.py' will be loaded, but not shown in the interface.

Exact steps for others to reproduce the error

1- Open new file, switch to bpy.context.scene.render.engine = 'BLENDER_WORKBENCH',
2- Save project, and close blender
3-Open blender on the previously saved .blend file
4-The icons or previews will be loaded but not shown in the panel

To make it work again:

1-switch to bpy.context.scene.render.engine = 'BLENDER_EEVEE' or 'CYCLES'
2-save project, close blender
3-reopen blender on the desired file