fix(sell): handle sell-balance stream errors instead of silently stopping

lib/screens/sell/cubits/sell_balance/sell_balance_cubit.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:developer' as developer;
 
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:realunit_wallet/models/balance.dart';
@@ -19,7 +20,21 @@ class SellBalanceCubit extends Cubit<Balance> {
           asset: appStore.apiConfig.asset,
         ),
       ) {
-    _subscription = _repository.watchBalance(state).listen(emit);
+    // Register an onError handler so a balance-stream error is logged instead
+    // of escaping as an unhandled async error that silently stops the sell
+    // balance (issue #657 P4 S6). cancelOnError defaults to false, so the
+    // subscription stays alive and later balances still update.
+    _subscription = _repository.watchBalance(state).listen(
+      emit,
+      onError: (Object error, StackTrace stackTrace) {
+        developer.log(
+          'sell balance stream error',
+          name: '$SellBalanceCubit',
+          error: error,
+          stackTrace: stackTrace,
+        );
+      },
+    );
   }
 
   final BalanceRepository _repository;

test/screens/sell/cubits/sell_balance_cubit_test.dart

@@ -75,6 +75,31 @@ void main() {
       expect(cubit.state.balance, BigInt.from(7000));
     });
 
+    test(
+        'a stream error is handled (not unhandled) and later balances still '
+        'update (issue #657 P4 S6 regression)', () async {
+      final cubit = SellBalanceCubit(repo, appStore);
+
+      // Without an onError handler this error escaped as an unhandled async
+      // error (failing the test) and stopped the sell balance. It must now be
+      // logged, and — cancelOnError being false — a later balance still lands.
+      controller.addError(Exception('balance backend blip'));
+      await Future<void>.delayed(Duration.zero);
+
+      final updated = Balance(
+        chainId: realUnitAsset.chainId,
+        contractAddress: realUnitAsset.address,
+        walletAddress: _wallet,
+        balance: BigInt.from(4242),
+        asset: realUnitAsset,
+      );
+      final ready = cubit.stream.firstWhere((b) => b.balance == BigInt.from(4242));
+      controller.add(updated);
+      await ready.timeout(const Duration(seconds: 1));
+
+      expect(cubit.state.balance, BigInt.from(4242));
+    });
+
     test('close() cancels the subscription cleanly', () async {
       final cubit = SellBalanceCubit(repo, appStore);