[firebase_app_check_web] Unhandled JS promise rejection on background token refresh network failure

[davidmigloz]

Bug description

When Firebase AppCheck's JS SDK performs automatic background token refresh and the network is unavailable, it throws an unhandled promise rejection. In a Flutter web app, this surfaces as a JSObject: FirebaseError that reaches runZonedGuarded as a fatal unhandled error. The Dart firebase_app_check wrapper has no mechanism to intercept or suppress this.

Error

JSObject: FirebaseError: AppCheck: Fetch failed to connect to a network.
Check Internet connection. Original error: Load failed. (appCheck/fetch-network-error).

Context

Chrome Mobile iOS, app backgrounded, single transient network blip. Firebase recovers automatically on retry, so the error is purely noise — but it's reported as fatal by the Dart runtime.

Steps to reproduce

1. Initialize Firebase AppCheck with activate(webProvider: ReCaptchaV3Provider(...)) in a Flutter web app
2. Lose network connectivity (e.g., toggle airplane mode, or the browser suspends network when the app is backgrounded on mobile)
3. Wait for the AppCheck SDK's automatic background token refresh to fire
4. Observe unhandled JSObject: FirebaseError reaching runZonedGuarded

Expected behavior

Background token refresh network failures should be handled internally by the SDK (retry silently) without throwing unhandled promise rejections into the host app's zone.

Possible fix

The firebase_app_check_web platform implementation could zone-guard the JS interop bridge to catch transient errors from the JS SDK's internal timer-based refresh, or surface them through a dedicated error stream/callback rather than letting them propagate as unhandled exceptions.

Workaround

Filter FirebaseError events containing fetch-network-error in Sentry's beforeSend hook (or equivalent error reporting filter).

Flutter doctor

Flutter 3.41.4 • Dart 3.11.1

Platform

Web (Chrome Mobile iOS 145.0.7632)

[Lyokone]

Hey, thanks for the detailed report!

Confirmed bug. The internal .listen() on the JS SDK's onTokenChanged stream in firebase_app_check_web was missing an onError handler. When the JS SDK's background token refresh hits a network error, the error propagates as an unhandled exception in the Dart zone instead of being caught.

[davidmigloz]

We've also observed a related error coming through the same path — appCheck/recaptcha-error instead of appCheck/fetch-network-error:

JSObject: FirebaseError: AppCheck: ReCAPTCHA error. (appCheck/recaptcha-error).

Same context: Flutter web, app backgrounded (lifecycle hidden), Chrome Mobile iOS. The background token refresh triggers a ReCAPTCHA re-verification which fails because the tab is suspended.

Your fix in #18135 should cover this as well since the onError handler on onTokenChanged is generic. Just flagging it for awareness in case there are other code paths where this could surface.