"ArgumentNullException" is thrown when clicking "Upscale" and "Generate PBR" buttons in the AI Assistant Materials Generator

How to reproduce:
1. Create a new Unity project
2. Go to AI -> Agree and install Unity AI -> Generate New -> Material
3. Go to the "Upscale" tab and click "Upscale"
4. Observe the Console

Expected results: No errors are thrown
Actual results: "ArgumentNullException" is thrown

Reproducible with: 2.7.0-pre.2
Not reproducible with: No other environment tested

Reproducible on: macOS Tahoe 26 (M1)
Not reproducible on: No other environment tested

First lines of errors:

• ArgumentNullException: Value cannot be null.
  Parameter name: imageStream
• ArgumentNullException: Generation request failed and download cannot proceed.
  Parameter name: arg