Steps to reproduce:
1. Create new Unity project
2. Create Filter Function Definition Asset in Project window
3. Open UI Builder window
4. Add Visual Element in Hierarchy window
5. Add Custom filter and assign New Filter Definition asset for Visual Element
6. Observe the Console window

Actual results: “NullReferenceException” error is thrown in the Console when assigning Filter Function Definition Asset in custom Filter
Expected results: No errors are thrown in the Console window

Reproducible with versions: 6000.3.13f1, 6000.4.3f1, 6000.5.0b4, 6000.6.0a3
Can’t test with versions: 6000.0.73f1 (Filter section not available, unable to create Filter Function Definition)

Tested on (OS): macOS Silicon Tahoe 26.4 (M1)

NullReferenceException: Object reference not set to an instance of an object
Unity.UIToolkit.Editor.FilterFunctionListViewItem.OnCustomValueChanged (UnityEngine.UIElements.ChangeEvent`1[T] evt) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Hierarchy/Selection/Editor/Controls/FilterFunctionListViewItem.cs:158)
UnityEngine.UIElements.EventBase`1[T].InvokeCallback (System.Delegate userCallback) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Events/EventBase.cs:740)
UnityEngine.UIElements.EventNoArgValue.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.EventCallbackInternal c) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Events/EventCallback.cs:123)
UnityEngine.UIElements.EventCallbackRegistry+DynamicCallbackList.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target, UnityEngine.UIElements.EventCallbackRegistry registry) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Events/EventCallbackRegistry.cs:333)
UnityEngine.UIElements.EventCallbackRegistry.InvokeCallbacksBubbleUp (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Events/EventCallbackRegistry.cs:720)