To reproduce:

1. Download attached "ModelImport_New.zip" project and open in Unity
2. In Project window select "Okay.fbx" file, right-click it and press Reimport
3. Observe that the Unity Editor successfully reimports the file
4. n Project window select "Okay.fbx" file, right-click it and press Reimport

Expected results: Unity Editor successfully reimports the file
Actual results: Error is thrown in Console:

"Assertion failed on expression: 'it != transformToMark.end()'
(Filename: C:\buildslave\unity\build\Runtime/Animation/OptimizeTransformHierarchy.cpp Line: 414)"

Notes:
- This issue happens on both Windows and macOS
- The only difference between these files is metafile, containing a now missing Hierarchy item, removing this solves the problem
- Unchecking "Optimize Game Objects" also removes the Error
- Does not seem possible to remove this via the ModelImporter GUI, only by hand in the Metafile
- Would expect a missing path to not cause an error while importing

Reproduced on Unity 2017.4.0f1, 2017.4.32f1, 2018.4.8f1, 2019.2.5f1, 2019.3.0b3 and 2020.1.0a4