When trying to set public ParentConstraint source with code, Unity crashes on play mode, if ParentConstraint is static, Unity doesn't crash.

To reproduce:
1. Create a new project
2. Import attached script "testing.cs"
3. Create an empty object and attach the script to it
4. Enter play mode

Expected result: play mode is entered without issues
Actual result: Unity crashes

Reproduced on:
2018.1.2f1 2018.2.0b6 2018.3.0a1

Could not test on 2017.4.x since ParentConstraint class does not exist there