Steps to reproduce:
1. Open attached Unity project “AnimationTypeIssue” from Google Drive link
2. In Project window go to Assets > Kevin Iglesias > Human Animations > Animations > Female > Combat > Spellcasting
3. Select any animation Asset
4. In Inspector > Import Settings select “Rig” tab
5. Change Animation Type to Legacy
6. Press “Apply” button
7. Observe the Console window
Actual results: Assertion failed on expression: 'MecanimDataWasBuilt()'
UnityEditor.GUIView:ProcessEvent (UnityEngine.EntityId,intptr,bool&)” errors are thrown in the Console window after switching Animation Type to Legacy
Expected results: No errors are thrown in the Console after switching Animation Type to Legacy
Reproducible with versions: 6000.0.74f1, 6000.3.16f1, 6000.4.8f1, 6000.5.0b9, 6000.6.0a5
Tested on (OS): macOS Tahoe 26.4.1 (M1)