How to reproduce:
1. Create a new project
2. Open the Quick Search window (either by clicking on the magnifying glass icon or using ctrl + k shortcut)
3. In the search field enter '?'
4. Select the first option from the search results
5. Click on the 'Help' button in the Preview Inspector
6. Close the Quick Search window
7. Repeat the steps 2 to 5
8. Observe the Console

Expected result: No visible errors present, the help string appears in the search field
Actual result: 'NullReferenceException' error gets thrown in the Console, the help string doesn't appear in the search field

Reproducible with: Quick Search 3.0.0-preview.1, 3.0.0-preview.3 (2020.2.3f1), 2021.1.0a1, 2021.1.0b4
Cannot test with: Quick Search 1.5.4 (2018.4.31f1), 2.1.0-preview.4 (2020.2.3f1) (the Quick Search window doesn't have the Preview Inspector), 3.0.0-preview.1, 3.0.0-preview.3 (2019.4.19f1) (the package versions are not supported), 2021.1.0b5, 2021.2.0a3 (the Help button doesn't work after reopening the Quick Search window)

Notes:
- The error doesn't get thrown in 2021.1.0b5, 2021.2.0a3, however, after pressing the Help button the help string doesn't appear in the search field