Addressables AutoGroupGenerator window is missing a NullReference guard for AutoGroupGenerator Settings asset

Steps to reproduce:

1. Open the attached project
2. Observe that the AutoGroupGenerator window does not have a settings file assigned, but the Generate button is still active
3. Press the Generate Addressables Groups Automatically button

Actual results: Multiple Errors are thrown in the Console window, including a GUI Error and a NullReferenceException

Expected results: If the settings file isn't assigned, the Generate button should be disabled, or the exception should be handled gracefully, informing the user that the settings file isn't assigned

Reproducible with versions: 2.9.1(6000.0.79f1), 4.0.1(6000.0.79f1, 6000.7.0a5)

Tested on (OS): Ubuntu 24, macOS 26.6.2