NullReferenceException + ArgumentNullException thrown in Mesh Generator when generation fails due to no subscription

Steps to reproduce:

1. Open AI Assistant's Mesh Generator (without an active generation subscription)
2. Attempt to generate a new Mesh
3. Observe the Console

Actual results: Two errors are logged in sequence:

NullReferenceException: Object reference not set to an instance of an object Unity.AI.Mesh.Services.Stores.Actions.Backend.Generation.GenerateMeshesAsync  

ArgumentNullException: Generation request failed and download cannot proceed.
Parameter name: arg 

Expected results: When there's no active subscription, ideally the Generate button (or equivalent action) should be disabled or show an upfront notice before the user attempts generation at all - preventing the failed request from being sent in the first place.

Failing that, if generation is still attempted and fails due to "NoSubscription," only a single, user-facing message should appear in the Console

Error reason is 'NoSubscription' and no additional error information was provided (https://generators.ai.unity.com).
UnityEngine.Debug:Log (object) 

Reproducible with versions: 2.18.0-pre.2 (6000.7.0a5, 6000.5.7f1, 6000.3.23f1), 2.17.0-pre.1(6000.6.0b8)

Tested on (OS): MacBook Pro - Tahoe 26.6.1