How to reproduce:
1. Open the attached project "OverlayIcon"
2. Open the "SampleScene" scene
3. Select the "GameObject" in the Hierarchy
4. In the Scene View, in the Tools Overlay, select the "OT" tool
5. Observe the added button on the Overlay Menu
Expected results: There is a default icon because the custom icon is not defined
Actual results: The button has no icon
Reproducible with: 6000.0.56f1, 6000.1.16f1, 6000.2.2f1, 6000.3.0a5
Reproducible on: macOS Sequoia 15.5 (M1)
Not reproducible on: No other environment tested