Steps to reproduce:

1. Create or open any Project with the UI Toolkit installed
2. Create a "UI Document" Asset and open it
3. Select the ".uxml" file in the Hierarchy
4. In the UI Builders Inspector enable "Canvas Background"
5. Add any Element in the Hiearchy and do not save the UI Document
6. Switch the "Canvas Background" to "Image" or "Camera" and save the UI Document
7. Try clicking on any of the Elements in the Viewport or the Hierarchy

Actual results:

UI Builder window becomes unusable, Elements cannot be manipulated in the Viewport or Hierarchy, UI Builder Windows has to be reopened

Error "NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UIElements.VisualTreeAsset.RemoveElementAndDependencies" is logged in Console every time Elements are clicked either in the Viewport or the Hierarchy

Expected results: No Errors are thrown

Reproducible with versions: 2023.3.0a6, 6000.0.13f1

Not reproducible with versions: 2021.3.40f1, 2022.3.40f1, 2023.3.0a5

Tested on (OS): macOS Sonoma, Windows 11

Error message:

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UIElements.VisualTreeAsset.RemoveElementAndDependencies (UnityEngine.UIElements.VisualElementAsset asset) (at <aff7113d094045d8bc63c60169e8e899>:0)
Unity.UI.Builder.BuilderAssetUtilities.RemoveElementFromSelectionInAsset (Unity.UI.Builder.BuilderDocument document, UnityEngine.UIElements.VisualElement ve) (at <dfb6f584aa494374b9127edd7bcf5d6d>:0)
Unity.UI.Builder.BuilderSelection.Select (Unity.UI.Builder.IBuilderSelectionNotifier source, UnityEngine.UIElements.VisualElement ve) (at <dfb6f584aa494374b9127edd7bcf5d6d>:0)
Unity.UI.Builder.BuilderViewport.OnPick (UnityEngine.UIElements.MouseDownEvent evt) (at <dfb6f584aa494374b9127edd7bcf5d6d>:0)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.EventCallbackRegistry+DynamicCallbackList.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.EventDispatchUtilities.HandleEventAcrossPropagationPathWithCompatibilityEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.EventBase compatibilityEvt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target, System.Boolean isCapturingTarget) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.EventDispatchUtilities.DispatchToCachedElementUnderPointerOrPanelRoot (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, System.Int32 pointerId, UnityEngine.Vector2 position) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.EventDispatchUtilities.DispatchToCapturingElementOrElementUnderPointer (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, System.Int32 pointerId, UnityEngine.Vector2 position) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.PointerEventBase`1[T].Dispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <aff7113d094045d8bc63c60169e8e899>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <98b89b20e7fa498f86e3aee402e1f36f>:0)