Model importer doesn't allow creation of a new Humanoid Avatar from a model file automatically when switching avatar definition to "Create From This Model" through a script. User has to manually do this step to create a working human avatar definition.

Reproduction steps:
1. Open the attached project.
2. Open "AnimationPostProcessor.cs" in the “Scripts” -folder and set the "variable _case1" to be "true".
3. Use Project view to Reimport "Character_Case1_Rig.fbx" from inside of the “Rig” -folder (Right mouse button > Reimport).
4. Observe the Inspector Import Settings view.

Actual result:
- Unity fails to create a Human Avatar out of the model through the AnimationPostProcessor script.

Expected result:
- The file should now have a configured Human Avatar, created from the model automatically by script, without requiring the user to manually do it through the Import Settings view.

Reproduced with:
2017.1.5f1, 2017.2.3p2, 2017.4.8f1, 2018.1.9f1, 2018.2.1f1, 2018.3.0a6