call.hangup() Throws "trying to add null key" Error — Call Not Ending on Web

[PRedemption]

Title: call.hangup() Throws "trying to add null key" Error — Call Not Ending on Web

Describe the bug

When attempting to end an active call using call.hangup() in SIP UA (Dart), the method fails with a console error:
"Trying to add null key".
As a result, the call does not terminate on the web platform and continues running in the background. The issue is consistently reproducible in Chrome.

To Reproduce

1. Initiate any SIP UA call from the Flutter Web application.
2. Attempt to end the call using call.hangup().
3. Open browser console (F12 → Console).
4. The error "trying to add null key" appears, and the call remains active.

Expected behavior

The call should cleanly terminate when call.hangup() is invoked, without throwing errors, and the UI should reflect the ended call state.

System Information

Flutter SDK Version: (e.g., 3.3.2 — app uses sdk: '>=3.3.2 <4.0.0')
Target OS and Version: Web
Browser: Chrome
Host OS and Version: (e.g., macOS / Windows / Linux)

Additional context

The error only appears on the web implementation; mobile platforms behave normally. Issue seems related to SIP UA call termination handling on web.