How to reproduce:
1. Open user's attached project ("bugtest1.zip")
2. In the Assets folder select, "Backwards Rifle Run" animation
3. In the model preview window, click on "Avatar" icon and select "Other..."
4. In the opened window, click on "model-23.11.2018-00-21-13"
5. Inspect the animation preview window

Expected result: the model can be seen in the preview window
Actual result: the model does not appear in the preview window

Reproducible with 2018.3.3f1. 2019.1.0b1, 2019.2.0a3
Not reproducible with: 2017.4.19f1

Notes:
1. It doesn't reproduce every time, so you might need to re-open the project (Happens around 1/3 tries)
2. Dragging the model inside the preview window instead of going through the menu seems not to be affected and works without issues