NullReferenceException is thrown when in the Player and not in the Editor

Unity: UI Toolkit Graphics

6.3.X

6.2.X

6.1.X

6.0.X

How to reproduce:
1. Open the attached project "IN-78869.zip"
2. Build And Run the project
3. Click the "Start" button in the Player

Expected results: No errors are thrown
Actual results: "NullReferenceException: Object reference not set to an instance of an object" is thrown

Reproducible with: 6000.0.9f1
Could not test with: 2021.3.40f1, 2022.3.35f1 (compilation errors in the Console window)

Reproducible on: Windows 10 22H2
Not reproducible on: No other environment tested