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Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. Open UI Library window (right-click in Hierarchy > UI Toolkit > Standard Elements > UI Library)
3. In UI Library find “Two Pane Split View” and double click on it to create it
4. In “Open UI stage” pop-up press “Continue” and save UXML file
5. Once the files are created, observe the Console window
Actual results: “TwoPaneSplitView needs exactly 2 children.” errors are thrown in the Console window
Expected results: No errors are thrown in the Console window
Reproducible with versions: 6000.6.0a7
Can’t test with versions: 6000.6.0a6 and lower (No such feature to create UI Elements in Hierarchy window)
Tested on (OS): macOS Tahoe 26.4.1 (M1)
TwoPaneSplitView needs exactly 2 children. UnityEngine.UIElements.UIElementsRuntimeUtilityNative:UpdatePanels () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Native/UIElementsUtility.bindings.cs:18)
TwoPaneSplitView needs exactly 2 children. UnityEngine.UIElements.Panel:Repaint () Unity.UIToolkit.Editor.PanelElement:FrameUpdate () (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Controls/PanelElement/PanelElement+Rendering.cs:170) Unity.UIToolkit.Editor.VisualElementEditingStage:RequestRefresh () (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Stages/VisualElementEditingStage.cs:87) Unity.UIToolkit.Editor.MenuUtility:ExecuteAdd (Unity.UIToolkit.Editor.VisualElementEditingStage,System.Type,UnityEngine.UIElements.VisualElementAsset) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Menu/MenuUtility.cs:322) Unity.UIToolkit.Editor.MenuUtility:EnterStageAndAdd (Unity.UIToolkit.Editor.VisualTreeAssetEditingContext,System.Type,UnityEngine.UIElements.VisualElementAsset) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Menu/MenuUtility.cs:316) Unity.UIToolkit.Editor.MenuUtility:AddInAppropriatePanelRendererComponent (UnityEngine.GameObject,System.Type,bool) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Menu/MenuUtility.cs:138) Unity.UIToolkit.Editor.MenuUtility:AddElement (System.Type,bool,bool) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Menu/MenuUtility.cs:72) Unity.UIToolkit.Editor.MenuUtility:AddElementAsSibling (System.Type) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Menu/MenuUtility.cs:31) Unity.UIToolkit.Editor.UIElementsProvider:AddElementToVisualTreeAsset (UnityEditor.Search.SearchItem) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Search/UIElementsProvider.cs:324) UnityEditor.Search.HierarchySearchItemHandler:Unity.Hierarchy.IHierarchyEditorNodeTypeHandler.OnDoubleClick (Unity.Hierarchy.HierarchyView,Unity.Hierarchy.HierarchyNode modreq(System.Runtime.InteropServices.InAttribute)&) Unity.Hierarchy.HierarchyView:OnClickEvent (UnityEngine.UIElements.ClickEvent) (at /Users/bokken/build/output/unity/unity/Modules/Hierarchy/Managed/HierarchyView.cs:1552) UnityEditor.GUIView:ProcessEvent (UnityEngine.EntityId,intptr,bool&) (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUIView.cs:70)
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