Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. In Project window create Default Runtime Theme File (Assets > Create > UI Toolkit > Default Runtime Theme File)
3. Select DefaultRuntimeThemeFile.tss file in Project window
4. In Inspector window > Inherited Themes list press "+"
5. In Missing Reference add Default.tss file
6. Press "Apply"
7. Observe the Console window

Actual results: Errors are spammed in the Console window
Expected results: No errors should be thrown in the Console window, the file should not be available to be selected

Reproducible with versions: 6000.0.58f1, 6000.2.6f1, 6000.3.0b3, 6000.4.0a1

Tested on (OS): Windows 11

Errors:

- Assertion failed on expression: 'type != SubFilterType::Exact'
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

- Assertion failed on expression: '!subFilter.dbPath.empty() || subFilter.type != SubFilterType::Exact'
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

- An infinite import loop has been detected. The following Assets were imported multiple times, but no changes to them have been detected. Please check if any custom code is trying to import them:
Assets/UnityDefaultRuntimeTheme.tss - (Force Reimport)(modified date 2025-09-23T08:51:21.9342431Z)
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

- MissingReferenceException: The object of type 'UnityEditor.UIElements.StyleSheets.ThemeStyleSheetImporter' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object. Parameter name: source
UnityEngine.Object+MarshalledUnityObject.TryThrowEditorNullExceptionObject (UnityEngine.Object unityObj, System.String parameterName) (at <25ce29052dfe4e49a635a42f7798806b>:0)
UnityEngine.Bindings.ThrowHelper.ThrowArgumentNullException (System.Object obj, System.String parameterName) (at <25ce29052dfe4e49a635a42f7798806b>:0)
UnityEditor.AssetImporters.AssetImporterEditor.UpdateSavedData (UnityEngine.Object source) (at <5818bc53b12b4560b54110897827736e>:0)
UnityEditor.AssetImporters.AssetImporterEditor.SaveChanges () (at <5818bc53b12b4560b54110897827736e>:0)
UnityEditor.AssetImporters.AssetImporterEditor.ApplyButton () (at <5818bc53b12b4560b54110897827736e>:0)
UnityEditor.AssetImporters.AssetImporterEditor.OnApplyRevertGUI () (at <5818bc53b12b4560b54110897827736e>:0)
UnityEditor.AssetImporters.AssetImporterEditor.ApplyRevertGUI () (at <5818bc53b12b4560b54110897827736e>:0)
UnityEditor.UIElements.StyleSheets.ThemeStyleSheetImporterEditor.OnInspectorGUI () (at <5818bc53b12b4560b54110897827736e>:0)
UnityEditor.UIElements.InspectorElement+<>c_{_}DisplayClass80_0.<CreateInspectorElementUsingIMGUI>b{_}_1 () (at <5818bc53b12b4560b54110897827736e>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)