Session crash: Cannot read properties of null (reading 'awaitActivePage') during act() after CDP activity

[vpendyal73]

Description

Stagehand 3.20+ crashes with Cannot read properties of null (reading 'awaitActivePage') during act() calls after rapid CDP interactions. The crash occurs non-deterministically but frequently when running UI automation loops that perform many sequential Accept/Reject clicks.

Steps to Reproduce

1. Start a Stagehand session
2. Navigate to a complex single-page application (Salesforce Lightning in our case)
3. Execute a loop that performs rapid act() calls (e.g., click Accept buttons in a list)
4. After 5-15 minutes of continuous activity, the session crashes

Environment

• Stagehand version: 3.21.0, 3.22.0 (crash), 3.19.5 (stable)
• OS: macOS 15.5 (Darwin 25.5.0)
• Python: 3.14.6
• Browser: Chromium (Stagehand-bundled)

Error Log

12:38:21 INFO HTTP Request: POST http://127.0.0.1:52802/v1/sessions/.../act "HTTP/1.1 200 OK"
12:38:23 INFO HTTP Request: POST http://127.0.0.1:52802/v1/sessions/.../act "HTTP/1.1 500 Internal Server Error"

... Stagehand server log shows:
TypeError: Cannot read properties of null (reading 'awaitActivePage')
    at BrowserContext.<anonymous> (/.../stagehand-darwin-arm64:7:15833)

Workaround

Downgrade to Stagehand 3.19.5:

pip install stagehand==3.19.5

Version 3.19.5 is stable and can run the same workload for 30+ minutes without crashing.

Additional Context

The crash appears to be a CDP session race condition. After many rapid CDP interactions (act/observe cycles), the internal page reference becomes null. The awaitActivePage property access then fails.

This is blocking our UI test automation suite. Happy to provide more details or test patches.