Steps to reproduce:
1. Create new Unity project
2. Enable Hierarchy Integration in Project Settings (Edit > Project Settings > UI Toolkit > Enable Hierarchy Integration)
3. In Editor’s Hierarchy create Panel Renderer GameObject
4. In Inspector create a new Source Asset and open it
5. In opened UI Builder’s Hierarchy add Scroll View element
6. Enter Play Mode
7. In Editor’s Hierarchy window press on 3rd VisualElement (PanelRenderer > NewUXMLTemplate > VisualElement > VisualElement > VisualElement) 
8. Observe the Console window

Actual results: “ArgumentOutOfRangeException” error is thrown in the Console window
Expected results: No errors are thrown in the Console window

Reproducible with versions: 6000.5.0a1 (0136d7329743), 6000.5.0b4, 6000.6.0a3
Not reproducible with versions: 6000.4.0a3 6000.4.3f1 (39d1a88d4dd1)

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

ArgumentOutOfRangeException: index of -1 should be in the range of 0 and 63 inclusively.
Parameter name: index
UnityEngine.UIElements.ToggleButtonGroupState.set_Item (System.Int32 index, System.Boolean value) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Controls/ToggleButtonGroup/ToggleButtonGroupState.cs:78)
UnityEngine.UIElements.EnumToggleField`1[T].SetValueWithoutNotify (T newValue) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Controls/StyleField/EnumToggleField.cs:132)
Unity.UIToolkit.Editor.StylePropertyField`3[TStyleValue,TValueField,TValue].SetValueWithoutNotify (TStyleValue newValue) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Hierarchy/Selection/Editor/Controls/StylePropertyField/StylePropertyField.cs:221)
UnityEngine.UIElements.BaseField`1[TValueType].set_value (TValueType value) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Controls/InputField/BaseField.cs:222)
Unity.UIToolkit.Editor.StylePropertyBinding.Update[TInline,TComputed] (UnityEngine.UIElements.BindingId& id, UnityEngine.UIElements.StylePropertyData`2[TInline,TComputedValue] value, Unity.UIToolkit.Editor.StyleInspectorElement+AuthoringContext authoringContext, UnityEngine.UIElements.VisualElement targetElement) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Inspector/Bindings/StylePropertyBinding.cs:560)
Unity.UIToolkit.Editor.StylePropertyBinding.Update (UnityEngine.UIElements.BindingId& id, UnityEngine.UIElements.StyleSheets.StylePropertyId propertyId, Unity.UIToolkit.Editor.StyleInspectorElement+AuthoringContext authoringContext, UnityEngine.UIElements.VisualElement targetElement) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Inspector/Bindings/Generated/StylePropertyBinding.gen.cs:121)
Unity.UIToolkit.Editor.StylePropertyBinding.Update (UnityEngine.UIElements.BindingContext& context) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Inspector/Bindings/StylePropertyBinding.cs:312)
UnityEngine.UIElements.BindingUpdater.UpdateUI (UnityEngine.UIElements.BindingContext& context, UnityEngine.UIElements.CustomBinding customBinding) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Bindings/BindingUpdater.cs:190)
UnityEngine.UIElements.BindingUpdater.UpdateUI (UnityEngine.UIElements.BindingContext& context, UnityEngine.UIElements.Binding bindingObject) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Bindings/BindingUpdater.cs:123)
UnityEngine.UIElements.VisualTreeDataBindingsUpdater.Update () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Bindings/VisualTreeDataBindingUpdater.cs:225)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase phase) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/VisualTreeUpdater.cs:146)
UnityEngine.UIElements.Panel.UpdateDataBinding () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Panel.cs:1518)
UnityEngine.UIElements.Panel.TickSchedulingUpdaters () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Panel.cs:1540)
UnityEngine.UIElements.Panel.UpdateForRepaint () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Panel.cs:1569)
UnityEditor.UIElements.EditorPanel.UpdateForRepaint () (at /Users/bokken/build/output/unity/unity/Modules/UIElementsEditor/EditorPanel.cs:98)
UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Panel.cs:1651)
UnityEditor.UIElements.EditorEventDispatchUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.Event eventInstance) (at /Users/bokken/build/output/unity/unity/Modules/UIElementsEditor/EditorEventDispatchUtility.cs:61)
UnityEditor.UIElements.DefaultWindowBackend.ProcessEvent (UnityEngine.Event e) (at /Users/bokken/build/output/unity/unity/Modules/UIElementsEditor/WindowBackends/DefaultWindowBackend.cs:114)
UnityEditor.GUIView.ProcessEvent (UnityEngine.EntityId entityId, System.IntPtr nativeEventPtr, System.Boolean& result) (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUIView.cs:70)
UnityEngine.StackTraceUtility:ExtractStringFromExceptionInternal(Object, String&, String&) (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:50)

Notes: on 6000.4 and 6000.5 streams use Panel Renderer GameObject