Repro:

1. Create a new project.
2. Create UI Document in Hierarchy. (GameObject -> UI Toolkit -> UI Document)
3. Import attached UXML file.
4. Attach imported UXML to UI document's 'Source Asset' field.
5. Delete the imported UXML file in project view.
6. Delete the UI document in hierarchy.

Expected Outcome: Errors stop filling up the console.
Actual Outcome: Errors reach 999+ within seconds after clearing the console.

Reproducible in 2022.2.5f1, tested on Windows 10.
Could not reproduce with UI Toolkit 1.0.0-preview.18 and 2020.3.41f1.

Have not extensively regression tested the issue - please contact the assigned ticket reporter if you need further testing or information.

Error Message:
GIF of console errors attached to ticket.
{code:java}
MissingReferenceException: The object of type 'VisualTreeAsset' 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.
UnityEngine.Object.get_name () (at <f45a99ffbb9f4d0580258c77e560b3ab>:0)
UnityEngine.UIElements.VisualTreeAsset.Instantiate () (at <91595155ebb442339abb5402bb548a5e>:0)
UnityEditor.UIElements.VisualTreeAssetEditor.UpdatePreviewTexture (System.Int32 width, System.Int32 height) (at <6ac5e7e1619a4ba7b29c228ba4ede87d>:0)
UnityEditor.UIElements.VisualTreeAssetEditor.Update () (at <6ac5e7e1619a4ba7b29c228ba4ede87d>:0)
UnityEditor.EditorApplication.Internal_CallUpdateFunctions () (at <00d63564201b48698d51680c10078897>:0)
{code}