Steps to reproduce:
1. Create new Unity project
2. Open UI Builder window
3. Add element to the Hierarchy (for example Toggle)
4. Delete the added element from Hierarchy
5. Don't save changes in UI Builder
6. Dock UI Builder window to Game tab

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Actual results:  Error is thrown -
{code:java}
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UIElements.Bindings.SerializedObjectBindingToBaseField`2[TValue,TField].OnFieldAttached () (at <8b75957d458a4d75a43ae082435fcb46>:0)
UnityEditor.UIElements.Bindings.SerializedObjectBindingBase.OnAttachToPanel (UnityEngine.UIElements.AttachToPanelEvent evt) (at <8b75957d458a4d75a43ae082435fcb46>:0)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <f00cec66743041849ac3a6c03ea57396>:0)
UnityEngine.UIElements.EventCallbackRegistry+DynamicCallbackList.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <f00cec66743041849ac3a6c03ea57396>:0)
{code}

Expected results: No errors thrown when docking UI Builder window with unsaved changes

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Reproducible with versions: 6000.0.54f1, 6000.1.13f1, 6000.2.0b11, 6000.3.0a3

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Tested on (OS): Windows 11

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Notes:

•  on 6000.2 and 6000.3 "No measure function set in this node's config

UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)" errors appear also