How to reproduce:
1. Open given Unity project
2. Go Window -> Custom -> Asset Bundle Builder
3. Press Build button

Scenario
This issue also occurs on applying modified Avtar mapping settings.

Steps to Reproduce.
1. Create a new project.
2. Import a 3D model.
3. Select imported model in project tab.
4.Rig > Configure > Mapping
5. Make some changes in Body of 3d model
6. Edit > Undo Avtar mapping modified or Ctrl/Cmd+Z
7. Select "Muscle & Settings"
8. Select "Apply" from "Unapplied import settings"

Expected result: Asset bundle build should be completed and it should be written in the Console
Actual result: Error is thrown

Regression first introduced in: 2017.3.0a1
Reproducible with: 2018.2.0b9, 2018.2.0b1, 2018.1.0f1, 2017.4.1f1, 2017.3.2f1
Not reproducible with: 2017.2.2p2, 2017.1.3p2

Fixed in: 2018.3.0a1