NullReferenceException is thrown when selecting "Custom Mesh" option in Shader Graph Main Preview

Steps to reproduce:

1. Create a new Unity project using a 3D Universal Template
2. Create and open any Shader Graph (e.g. Assets > Create > Shader Graph > Blank Shader Graph)
3. Right-click on the Main Preview window
4. Select the Custom Mesh option
5. Observe the Console

Actual results: "NullReferenceException: Object reference not set to an instance of an object" is thrown, and it is impossible to select a custom mesh

Expected results: No errors, and a pop-up window for mesh selection opens up

Reproducible with versions: 6000.4.0a2, 6000.4.0b3, 6000.5.0a4

Not reproducible with versions: 6000.0.64f1, 6000.3.3f1, 6000.4.0a1

Tested on (OS): Windows 11, macOS 26.1 (M1 Pro)