Reproduction steps:
1. Open the attached project "TimelineArgumentNullException.zip";
2. Open the "Main.unity" scene;
3. Make sure that the Timeline and Animation windows are opened;
4. Select the New Timeline object in the Inspector;
5. Right-click on the Cube object and select "Edit in the Animation window";
6. Delete the Cube object;
7. Press the red spot button on the Sphere to record;
8. Observe the error in the Console window.

Expected results: the other object will be recorded without any problems;
Actual results: after removing one object and recording another, the error pops up in the console:

ArgumentNullException: Argument cannot be null.
Parameter name: key

Reproduced with: 2017.1.0f1, 2017.1.0f3, 2017.2.0b3.