How to reproduce:
1. Create a new Unity project
2. Create an Animator Controller
3. Create two states and a transition between them
4. Select the transition and add a Condition
5. In the Animator window create a new Parameter
6. Check the Conditions of the previously created transition

Expected result: the conditions can be clicked on to select the newly created parameter
Actual result: the condition drop-down is unresponsive. Clicking on it throws a NullReferenceException

Reproducible with: 2019.1.0a10, 2019.1.0b4, 2019.2.0a6
Not reproducible with: 2017.4.21f1, 2018.3.7f1, 2019.1.0a9