Reproduction steps:
1. Open attached project
2. Click on the "ply_gn_warrior_00" prefab (in Assets\Models\ply_gn_warrior_00.fbx)
3. Click the animation tab
4. In Mask drop-down expand the Transform drop-down
5. Notice that the Transform drop-down is empty (check Screenshot.png)

Actual result: The Transform drop-down is empty.

Expected result: The Transform drop-down shouldn't be empty.

Note: When clicking the "Fix Mask" button and applying the changes the "Fix Mask" button is still displayed (mask wasn't fixed).

Regression introduced in: 5.3