[Timeline] Exceptions are thrown on resetting Signal Receiver component.

Exceptions are thrown continuously on resetting Signal Receiver component.
Steps to repro:
1. Create a new project.
2. GameObject > Create Empty.
3. Add "Signal Receiver" component from the inspector.
4. Click o Add Reaction > Cogwheel > Reset.

Actual Result:
The exceptions are thrown in the console.

Expected Result:
The Component should reset without any error.

Reproducible in:
2019.3.0a4, 2019.2.0b4, 2019.1.0f2.

Environment:
Windows & Mac.

NOTE: Signal Receiver is introduced in 2019.1.