Steps to reproduce:
1. Create a new Unity project using the Universal 3D Template
2. In the Hierarchy, create a Button object (Right-click > UI Toolkit > Standard Elements > Button)
3. In the "Open UI Stage" pop-up, press "Continue" and save the UXML file
4. Create a UI Animation Clip for the Button element
5. Enter Play Mode
6. Add any Property to the clip for Button element (e.g. Align Property)
7. Exit Play Mode
8. In the "UI Assets Have Been Modified" dialog, press "Save"
9. Observe the Console window

Actual results: “NullReferenceException” error is thrown in the Console window

Expected results: No errors are thrown in the Console window

Reproducible with versions: 6000.7.0b3, 7000.0.0a1

Can’t test with versions: 6000.6.4f1 and lower (UI Animation feature is not available)

Tested on (OS): macOS 27 (Apple Silicon)

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UIElements.ResolvedStyleAccess.get_animationNames () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Style/Generated/ResolvedStyleAccess.gen.cs:23)
UnityEngine.UIElements.IResolvedStyle.get_unityAnimationClip () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Style/AnimationClipStyleCompat.cs:64)
Unity.UIToolkit.Editor.VisualElementAnimationClipUtility.FindClipOwner (UnityEngine.UIElements.VisualElement element) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/AnimationWindow/VisualElementAnimationClipUtility.cs:15)
Unity.UIToolkit.Editor.SceneVisualElementAnimationResponder.TryEditVisualElement (UnityEditor.AnimationWindow window, UnityEngine.UIElements.VisualElement element, System.Boolean inStage, UnityEditor.IAnimationWindowSelectionItem& newSelection) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Recording/SceneVisualElementAnimationResponder.cs:80)
Unity.UIToolkit.Editor.SceneVisualElementAnimationResponder.OnSelectionChange (UnityEditor.AnimationWindow window, UnityEngine.Object selectedObject, UnityEditor.IAnimationWindowSelectionItem& newSelection) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Recording/SceneVisualElementAnimationResponder.cs:27)
UnityEditor.AnimationWindow.OnSelectionChangeInternal (System.Boolean fromCallback) (at /Users/bokken/build/output/unity/unity/Modules/AnimationWindow/Editor/AnimationWindow.cs:270)
UnityEditor.AnimationWindow.OnSelectionChange () (at /Users/bokken/build/output/unity/unity/Modules/AnimationWindow/Editor/AnimationWindow.cs:241)
UnityEditor.HostView.OnSelectionChange () (at /Users/bokken/build/output/unity/unity/Editor/Mono/HostView.cs:475)
UnityEngine.StackTraceUtility:ExtractStringFromExceptionInternal(Object, String&, String&) (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:50)