"No GUI Implemented" is displayed when using PhysicsMask with a CustomEditor

1. Open the attached "IN-143007_Empty Project" project
2. Open the "SampleScene" (Assets/Scenes/SampleScene)
3. Select the "GameObject" in the Hierarchy
4. Observe the Inspector

Actual result: "No GUI Implemented" is displayed in the Inspector window instead of the editable properties
Expected result: The PhysicsMask struct should render properly in the Inspector window

Reproducible with: 6000.3.0a5, 6000.3.16f1, 6000.4.8f1, 6000.5.0b9, 6000.6.0a5
Could not test with: 2023.1.0a1, 6000.0.75f1, 6000.3.0a4 (PhysicsMask struct didn't exist until later 6000.3 versions)

Reproducible on: Windows 11
Not reproducible on: No other environments tested