Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/BaseAssets/Scenes/MainMenu.unity” Scene
3. Enter the Play Mode
4. Exit the Play Mode
5. Keep the Editor open. In the OS File Explorer, navigate to “Assets/BaseAssets/UI/ThemeStyles” and delete all contents
6. Copy the files from the provided “UpdatedFiles” folder into “Assets/BaseAssets/UI/ThemeStyles”
7. Refocus the Unity Editor and allow it to recompile/refresh
8. Repeat step 3
9. Observe the Console

Expected result: No Errors exist and Editor enters the Play Mode
Actual result: Error “MissingReferenceException: The object of type 'UnityEngine.UIElements.ThemeStyleSheet' has been destroyed but you are still trying to access it“ is present

Reproducible with: 6000.3.4f1, 6000.4.0b4
Could not test with: 6000.0.65f1, 6000.5.0a4 (Library\PackageCache\com.unity.physics@4ad4f3485fc7\Unity.Physics\ECS\Base\Aspects\ColliderAspect.cs(14,53): error CS0246: The type or namespace name 'IAspect' could not be found (are you missing a using directive or an assembly reference?))

Reproducible on:
Play Mode

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested

Note:
- Forcing EditorSettings.refreshImportMode to AssetDatabase.RefreshImportMode.InProcess does not resolve the issue. The only workaround is restarting the Editor