How to reproduce:
1. Create a new project using the “Universal 3D” template
2. In the menu bar, select “Edit → Preferences → General → New Hierarchy” and ensure that the “Use new Hierarchy window” option is enabled
3. In the new Hierarchy window, select a GameObject
4. While the GameObject is selected, hold the left mouse button down for a second on an open space in the new Hierarchy window
5. Observe the Console
Actual result: “NullReferenceException: Object reference not set to an instance of an object” error is thrown in the Console
Expected result: No errors are thrown in the Console
Reproducible with: 6000.4.0a2 (417aa67423b3), 6000.4.1f1
Not reproducible with: 6000.3.12f1, 6000.4.0a1, 6000.5.0b1, 6000.6.0a1
Could not test with: 6000.0.71f1 (New Hierarchy window is not available)
Reproducible on: Windows 11
Not reproducible on: No other environments tested