Reproduction steps:
1. Open the attached project (UIToolkitBug.zip)
2. Open scene Scenes/SampleScene
2. Press the Play button
3. In the Game view, press the "Test" button
4. In the Project window, right-click and reimport Assets/Main.cs
5. In the Game view, press the "Test" button
6. In the Hierarchy window, select the GameObject "Main"
7. In the Inspector window, under the component "Main (Script)", enable the "Fix Reload" field
8. In the Project window, right-click and reimport Assets/Main.cs
9. In the Game View, press the "Test" button

Expected result: the button can be pressed in all three instances
Actual result: NullReferenceException error is thrown after enabling "Fix Reload" and reimporting Asset

Reproducible with: 1.0.0-preview.15, 1.0.0-preview.16, 1.0.0-preview.17 (2020.3.17f1, 2021.1.20f1)
Could not test with: 2019.4.30f1 (UI Toolkit package unavailable), 2021.2.0b9, 2022.1.0a7 (cannot resolve errors after upgrading the attached project: "error CS0234: The type or namespace name does not exist in the namespace...")