"NullReferenceException" error is thrown when calling UIDocument.runtimePanel in OnEnable with ExecuteInEditMode

Reproduction steps:
1. Open the “ReproProject“ project
2. Open the “Assets/Scenes/SampleScene“ scene
3. Enter Play Mode
4. Observe the Console window

Expected result: No errors are thrown in the Console window
Actual result: “NullReferenceException: Object reference not set to an instance of an object“ error is thrown in the Console window

Reproducible with: 6000.0.2f1
Couldn’t test with: 2021.3.38f1, 2022.3.28f1 (due to UIDocument.runtimePanel not existing)

Reproducible on: Windows 11 Pro (22H2)
Not reproducible on: No other environment tested