NullReferenceException is thrown when saving a UI Toolkit element animation with Animation window Preview enabled

How to reproduce:

1. Open the attached “IN-154909” project
2. Open the “Sample Scene” scene
3. In the Hierarchy tab, expand “PlayerHUD” > “#Header” > “#HeaderLeft” and select the “#SpinCoinIcon” VisualElement
4. Open the “Animation” window (Ctrl + 6)
5. In the top bar of the “Animation” window near the record button, enable “Preview” mode
6. In the left part of the “Animation” window, expand “Background Image”
7. Change “Background Image.gradient.angle” value to 3 and save the changes (Ctrl + S)
8. In the Hierarchy tab, unselect and select “#SpinCoinIcon” VisualElement
9. In the “Animation” window, repeat steps 6 and 7, change “Background Image.gradient.angle” value to 4 and save the changes (Ctrl + S)
10. Observe the result in the Console

Actual result: The “NullReferenceException: Object reference not set to an instance of an object” error is thrown
Expected result: The animation is saved without errors

Reproducible with: 6000.6.0a6, 6000.6.4f1, 6000.7.0b3, 7000.0.0a1
Could not test with: 6000.0.1f1, 6000.0.85f1, 6000.3.25f1, 6000.6.0a5 (Animating UI Toolkit elements in the Animation window is not available)

Reproducible on: Windows 11 Enterprise (25H2)
Not reproducible on: No other environments tested

Notes:

• Any property in the “Animation” window could be changed to any value to reproduce the bug
• The “Animation” window could also be opened without a hotkey (“Window” > “Animation” > “Animation”)
• In the 6000.6.0a6 and 6000.6.4f1, step 7 could be skipped (in 6.6 “Background Image” is absent)
• In 6000.6, “Enable UI Toolkit Animation” and “Enable in-scene UI Authoring” (Project Settings > UI Toolkit) must be enabled, and the clip must be assigned with -unity-animation-clip instead of animation-name