To reproduce:

1. Download attached project "UnityTimelineBugs2.zip" and open in Unity
2. Open "TimelineMissingClipTest" scene
3. Select "Boy <---" > "Boy <---" > "Timeline <---" gameObject i Hierarchy window
4. Open Timeline window and observe the triangle warning sign
5. Click on the triangle warning sign and try to point it to different animation clip

Expected result: You can easily point to different animation clip
Actual result: You could not point to different animation clip because Inspector window fields are greyed out

Notes:
- This issue reproduces on both Windows and OSX
- More information and pictures - https://forum.unity.com/threads/unable-to-fix-missing-animation-clip.537207/

Reproduced on Unity 2017.1.4p2, 2017.2.3p2, 2017.3.2f1, 2017.4.6f1, 2018.1.7f1, 2018.2.0b11 and 2018.3.0a3