Steps to reproduce:

1. Open the UI Debugger
2. Attempt to assign a custom shader

Result

Invalid value for property '-unity-material'
UnityEngine.Debug:ExtractStackTraceNoAlloc (byte*,int,string)
UnityEngine.StackTraceUtility:ExtractStackTrace () (at D:/UnitySrc/u0/Runtime/Export/Scripting/StackTrace.cs:35)
UnityEngine.DebugLogHandler:Internal_Log (UnityEngine.LogType,UnityEngine.LogOption,string,UnityEngine.Object)
UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
UnityEngine.Logger:Log (UnityEngine.LogType,object)
UnityEngine.Debug:LogError (object)
UnityEditor.UIElements.Debugger.StyleField:SetPropertyValue (object,int) (at D:/UnitySrc/u0/Modules/UIElementsEditor/Debugger/StylePropertyDebugger.cs:735)
UnityEditor.UIElements.Debugger.StyleField:<GetOrCreateField>b__13_0<UnityEditor.UIElements.ObjectField, UnityEngine.Object> (UnityEngine.UIElements.ChangeEvent`1<UnityEngine.Object>) (at D:/UnitySrc/u0/Modules/UIElementsEditor/Debugger/StylePropertyDebugger.cs:581)
UnityEngine.UIElements.EventCallbackFunctor`1<UnityEngine.UIElements.ChangeEvent`1<UnityEngine.Object>>:Invoke (UnityEngine.UIElements.EventBase) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventCallback.cs:64)
UnityEngine.UIElements.EventCallbackRegistry/DynamicCallbackList:Invoke (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel,UnityEngine.UIElements.VisualElement) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventCallbackRegistry.cs:246)
UnityEngine.UIElements.EventDispatchUtilities:HandleEvent_BubbleUpCallbacks (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel,UnityEngine.UIElements.VisualElement) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventDispatchUtilities.cs:405)
UnityEngine.UIElements.EventDispatchUtilities:HandleEventAcrossPropagationPath (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel,UnityEngine.UIElements.VisualElement,bool) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventDispatchUtilities.cs:212)
UnityEngine.UIElements.EventDispatchUtilities:PropagateEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel,UnityEngine.UIElements.VisualElement,bool) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventDispatchUtilities.cs:70)
UnityEngine.UIElements.EventDispatchUtilities:DefaultDispatch (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventDispatchUtilities.cs:558)
UnityEngine.UIElements.EventBase:Dispatch (UnityEngine.UIElements.BaseVisualElementPanel) (at D:/UnitySrc/u0/Modules/UIElements/Core/Events/EventBase.cs:115)
UnityEngine.UIElements.EventDispatcher:ProcessEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel) (at D:/UnitySrc/u0/Modules/UIElements/Core/EventDispatcher.cs:419)
UnityEngine.UIElements.EventDispatcher:ProcessEventQueue () (at D:/UnitySrc/u0/Modules/UIElements/Core/EventDispatcher.cs:365)
UnityEngine.UIElements.EventDispatcher:OpenGate () (at D:/UnitySrc/u0/Modules/UIElements/Core/EventDispatcher.cs:318)
UnityEngine.UIElements.EventDispatcherGate:Dispose () (at D:/UnitySrc/u0/Modules/UIElements/Core/EventDispatcher.cs:79)
UnityEngine.UIElements.EventDispatcher:ProcessEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel) (at D:/UnitySrc/u0/Modules/UIElements/Core/EventDispatcher.cs:433)
UnityEngine.UIElements.EventDispatcher:Dispatch (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel,UnityEngine.UIElements.DispatchMode) (at D:/UnitySrc/u0/Modules/UIElements/Core/EventDispatcher.cs:215)
UnityEngine.UIElements.BaseVisualElementPanel:SendEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.DispatchMode) (at D:/UnitySrc/u0/Modules/UIElements/Core/Panel.cs:654)
UnityEngine.UIElements.UIElementsUtility:DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel) (at D:/UnitySrc/u0/Modules/UIElements/Core/UIElementsUtility.cs:545)
UnityEngine.UIElements.UIElementsUtility:UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (int,intptr,bool&) (at D:/UnitySrc/u0/Modules/UIElements/Core/UIElementsUtility.cs:244)
UnityEngine.UIElements.UIEventRegistration:ProcessEvent (int,intptr) (at D:/UnitySrc/u0/Modules/UIElements/Core/UIElementsUtility.cs:79)
UnityEngine.UIElements.UIEventRegistration/<>c:<.cctor>b__1_2 (int,intptr) (at D:/UnitySrc/u0/Modules/UIElements/Core/UIElementsUtility.cs:33)
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&) (at D:/UnitySrc/u0/Modules/IMGUI/GUIUtility.cs:224)