How to reproduce:
1. Open the user’s attached “My project (13).zip” project
2. Open “MyDocument.uxml” file
3. In the UI Builder, select the “Label” element
4. Open the “MyDocument.uxml” file with a text editor program
5. Make a change and save the file
6. Focus UI Builder window
7. Observe the Console window

Expected result: “ArgumentNullException: Value cannot be null. Parameter name: property” error is logged once
Actual result: “ArgumentNullException: Value cannot be null. Parameter name: property” error is spammed

Reproducible with: 2023.3.0a9. 2023.3.0b1
Not reproducible with: 2021.3.33f1, 2022.3.16f1, 2023.2.4f1, 2023.3.0a8

Reproduced on: macOS 13.6.3 (Intel)
Not reproducible on: No other environment tested

Note: not reproducible if focusing the editor window in the 6th step