Have not tried to reproduce - but the following was the steps that I performed just before the crash.

To reproduce:
- Download the "Mecanim Example Scenes" from Asset Store
- Open "Generic Skeleton Example" scene
- Select "mine_bot" object in Hierarchy
- Open "MineBot" animation controller
- In the Animation Controller window, double-click on "Locotion"
- Select the "Loco" blend tree and a new Motion Field
- Un-tick the option for "Automate Thresholds"
- In the parameter for "Computer Thresholds", change between the different thresholds from the drop down
- Observe that the slider in blend tree after while gets broken and then a crash