How to reproduce:
1. Open the attached project
2. Make sure Animation and Inspector windows are open
3. Select idle animation from Assets folder
4. Click on some random time so time indicator changes position
5. Click on "Add keyframe"
6. Undo this action (Ctrl + Z)
Actual result: Created key frames are deleted and error shows up ArgumentException: Getting control 7's position in a group with only 7 controls when doing ValidateCommand Aborting
Expected result: key frames are deleted and error doesn't show up

In step 5 if you create key frame with RMB click and Add Key, undoing this step generates a bit different error:
ArgumentException: GUILayout: Mismatched LayoutGroup.ValidateCommand

Note: You won't get error if Inspector window is not open

Reproducible: 2017.1.0b6, 2017.2.0a1
Not reproducible: 5.6.1f1