How to reproduce:
1. Open case_1092857-Preview window repro
2. Go to Window -> Animation -> Animator
3. In the Project window select "New Animator Controller"
4. In the Animator window select the transition between "New State" and "New State 0"
5. Change Transition Duration to 0.05
6. In the Animator window select "New State"
7. Select the transition between "New State" and "New State 0" again
8. Change Transition Duration to 0.2

Expected result: Preview window of a transition works
Actual result: Preview window shows NaN

Reproducible with: 2017.4.16f1, 2018.1.9f2, 2018.2.19f1, 2018.3.0f1, 2019.1.0a11

Note: There are no console errors in this repro project, but there are in the one that the user has attached