To reproduce:

1. Open project, open scene "test", press "Load animation" button.

Notice nothing happens.

2. Uncomment line 40 in RuntimeControllerTest.cs

3. Repeat step 1 again.

The editor should now crash.

Note: This is a different bug than 726191, even though they have the same symptoms (and the same title). I've double-checked that it is not reproducible in 5.2.1p1, while this one is.