fix(document_scanner): prevent 'Reply already submitted' on duplicate activity result

[MilanJa]

Summary

Fixes IllegalStateException: Reply already submitted thrown from DocumentScanner.handleScanningResult (and the cancel/error paths) when the platform delivers onActivityResult more than once for the same START_DOCUMENT_ACTIVITY request code.

Fixes #857.

Root cause

DocumentScanner stores the caller's MethodChannel.Result in pendingResult but never clears the field after completing it:

// before
pendingResult?.success(resultMap)            // in handleScanningResult
pendingResult?.error(TAG, "Operation cancelled", null)  // in onActivityResult
// ...pendingResult still non-null, pointing at an already-completed reply

If the OS then redelivers the activity result — which ColorOS (Oppo) is observed to do across certain lifecycle transitions — the second onActivityResult invocation calls .success(...) / .error(...) on the already-completed MethodChannel.Result, and DartMessenger$Reply.reply throws IllegalStateException: Reply already submitted, crashing the host app.

Stack trace (abridged) from the linked issue:

Caused by java.lang.IllegalStateException: Reply already submitted
    at io.flutter.embedding.engine.dart.DartMessenger$Reply.reply(DartMessenger.java:425)
    at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler$1.success(MethodChannel.java:272)
    at com.google_mlkit_document_scanner.DocumentScanner.handleScanningResult(DocumentScanner.kt:...)
    at com.google_mlkit_document_scanner.DocumentScanner.onActivityResult(DocumentScanner.kt:...)
    at io.flutter.embedding.engine.FlutterEngineConnectionRegistry$FlutterEngineActivityPluginBinding.onActivityResult(...)

Fix

Introduce a consumePendingResult() helper that atomically returns the current pendingResult and nulls the field, and route every submission site through it:

• handleScanningResult (success path)
• onActivityResult RESULT_CANCELED / unknown result branches
• handleScanner error paths (invalid options, intent-sender failures)

A redelivered activity result now short-circuits via the null receiver (consumePendingResult()?.xxx(...)) instead of double-submitting.

private fun consumePendingResult(): MethodChannel.Result? {
    val reply = pendingResult
    pendingResult = null
    return reply
}

The change is confined to one file: packages/google_mlkit_document_scanner/android/src/main/kotlin/com/google_mlkit_document_scanner/DocumentScanner.kt (+20/-6).

Validation

• flutter build apk --debug on packages/example compiles cleanly against the patched Kotlin source.
• The user-facing crash cannot be reproduced on demand (it is OEM-specific timing triggered by ColorOS redelivery), but the mechanism is eliminated: after the first .success() / .error(), pendingResult is null and any subsequent callback is a no-op.

Suggested CHANGELOG entry (for maintainers)

## 0.4.2

* Fix `IllegalStateException: Reply already submitted` crash when `onActivityResult` is
  delivered more than once (observed on Oppo/ColorOS devices).

I did not bump the package version in this PR — leaving that to the maintainer release cadence.

Test plan

• flutter build apk --debug on packages/example — passes.
• Happy path smoke test on an Android device (maintainers).
• Cancel path smoke test (maintainers).

[github-actions]

This PR is stale because it has been open for 14 days with no activity.

[github-actions]

This PR was closed because it has been inactive for 14 days since being marked as stale.