fix(flutter): Allow empty associatedEventId (#4108)

SentryFeedbackForm asserted that associatedEventId is never
SentryId.empty(), but Hub.captureMessage/captureEvent return the
empty id whenever the capture is not sent (network error, disabled
hub, beforeSend drop). Opening the form with that id crashed debug
and profile builds.

Treat an empty id the same as null (no association) instead.

Fixes #4042

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: lucas-zimerman

packages/flutter/lib/src/feedback/sentry_feedback_form.dart

@@ -13,12 +13,16 @@ import '../utils/internal_logger.dart';
 
 /// A form for submitting user feedback to Sentry.
 class SentryFeedbackForm extends StatefulWidget {
+  /// An empty [associatedEventId] (e.g. returned when the original event
+  /// could not be captured) is treated as no association.
   SentryFeedbackForm({
     super.key,
-    this.associatedEventId,
+    SentryId? associatedEventId,
     this.screenshot,
     @internal Hub? hub,
-  }) : assert(associatedEventId != const SentryId.empty()),
+  }) : associatedEventId = associatedEventId == const SentryId.empty()
+           ? null
+           : associatedEventId,
        _hub = hub ?? HubAdapter() {
     assert(
       _hub.options is SentryFlutterOptions,

packages/flutter/test/feedback/sentry_feedback_form_test.dart

@@ -514,6 +514,48 @@ void main() {
         ),
       ).called(1);
     });
+
+    testWidgets(
+      'submits feedback without association when associatedEventId is empty',
+      (tester) async {
+        await fixture.pumpFeedbackForm(
+          tester,
+          (hub) => SentryFeedbackForm(
+            hub: hub,
+            associatedEventId: const SentryId.empty(),
+          ),
+        );
+
+        when(
+          fixture.hub.captureFeedback(
+            any,
+            hint: anyNamed('hint'),
+            withScope: anyNamed('withScope'),
+          ),
+        ).thenAnswer(
+          (_) async => SentryId.fromId('1988bb1b6f0d4c509e232f0cb9aaeaea'),
+        );
+
+        await tester.enterText(
+          find.byKey(ValueKey('sentry_feedback_message_textfield')),
+          "fixture-message",
+        );
+        await tester.tap(find.text('Send Bug Report'));
+        await tester.pumpAndSettle();
+
+        verify(
+          fixture.hub.captureFeedback(
+            argThat(
+              predicate<SentryFeedback>(
+                (feedback) => feedback.associatedEventId == null,
+              ),
+            ),
+            hint: anyNamed('hint'),
+            withScope: anyNamed('withScope'),
+          ),
+        ).called(1);
+      },
+    );
   });
 
   group('$SentryFeedbackForm localization', () {