"Animation Clip is Read-Only" is shown in the Animation window when nothing is selected

How to reproduce:
1. Create a new Unity project
2. Go to Window -> Animation -> Animation
3. Make sure no GameObject is selected and observe the results

Expected results: The left panel is empty because no GameObject is selected
Actual results: The left panel says "Animation Clip is Read-Only" even though nothing is selected

Reproducible with: 6000.3.0a1, 6000.3.12f1, 6000.4.2f1, 6000.5.0b2, 6000.6.0a2
Not reproducible with: 6000.0.71f1, 6000.2.0b2

Reproducible on: macOS Tahoe 26 (M1)
Not reproducible on: No other environment tested