How to reproduce:
1. Open attached "Test Environment 4 Assets" project
2. In Project view open Assets/Mayan Temple/Example/Boxman/Models/Boxman and select "boxman.fbx"
3. In Inspector open Rig tab and press button Configure...
4. Move some of the bones, so model is not in a T-Pose
5. In Inspector select Mapping/Clear and then Pose/T-Pose

Expected result: Avatar is placed into a T-Pose
Actual result: Editor crashes

Reproduced with: 2018.2.16f1, 2018.3.0b9, 2019.1.0a7
Does not reproduce with: 2017.4.15f1, 2018.2.12f1

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Update: fixed in 2019.1.0a8