To reproduce:

1. Open curve editor, select non-Linear transition type (I chose Step)
2. Remove or comment the line out of code
3. Curve Editor still displays the old state
4. Clicking transition type buttons doesn't change the transition type of selected item anymore, you need to drag the keyframe in the curve editor instead, then the editor will "reconnect" with code again and it will save the pre-step 2 easingCurve value with new value and frame values.