How to reproduce:
1. Create a new Unity project
2. Open Player Settings window and choose "XR Settings" tab
3. Notice that the Assets/XR folder is created
4. Select any other settings tab (or close the window completely)
5. Delete the Assets/XR folder
6. Inspect the console

Expected results: No errors are thrown
Actual results: "SerializedObject target has been destroyed." error is thrown

Reproducible with: 2019.2.8f1, 2019.3.0b6, 2020.1.0a7
Couldn't test with 2017.4, 2018.4 (No package manager/the preview package does not have "XR Settings" tab in Player Settings)
Reproducible with package versions: 3.0.2-preview.1, 3.0.4-preview.1
Does not reproduce with package versions: 2.0.0-preview.3, 3.0.1

Note: Creating/deleting new assets in the "Project" window throws more of the same errors