CW-441 xmr price issue (#1005)

* refactor fiat_rate_update

* remove commented line

lib/reactions/fiat_rate_update.dart

@@ -6,6 +6,7 @@ import 'package:cake_wallet/store/app_store.dart';
 import 'package:cake_wallet/store/dashboard/fiat_conversion_store.dart';
 import 'package:cake_wallet/store/settings_store.dart';
 import 'package:cw_core/wallet_type.dart';
+import 'package:mobx/mobx.dart';
 
 Timer? _timer;
 
@@ -15,7 +16,7 @@ Future<void> startFiatRateUpdate(
     return;
   }
 
-  _timer = Timer.periodic(Duration(seconds: 30), (_) async {
+  final _updateFiat = (_) async {
     try {
       if (appStore.wallet == null || settingsStore.fiatApiMode == FiatApiMode.disabled) {
         return;
@@ -33,5 +34,20 @@ Future<void> startFiatRateUpdate(
     } catch (e) {
       print(e);
     }
+  };
+
+  _timer = Timer.periodic(Duration(seconds: 30), _updateFiat);
+  // also run immediately:
+  _updateFiat(null);
+
+  // setup autorun to listen to changes in fiatApiMode
+  autorun((_) {
+    // restart the timer if fiatApiMode was re-enabled
+    if (settingsStore.fiatApiMode != FiatApiMode.disabled) {
+      _timer = Timer.periodic(Duration(seconds: 30), _updateFiat);
+      _updateFiat(null);
+    } else {
+      _timer?.cancel();
+    }
   });
 }

lib/view_model/dashboard/balance_view_model.dart

@@ -66,7 +66,8 @@ abstract class BalanceViewModelBase with Store {
     final price = fiatConvertationStore.prices[appStore.wallet!.currency];
 
     if (price == null) {
-      throw Exception('No price for ${appStore.wallet!.currency} (current wallet)');
+      // price should update on next fetch:
+      return 0;
     }
 
     return price;