Fix issues from code review

lib/view_model/send/send_view_model.dart

@@ -42,7 +42,8 @@ abstract class SendViewModelBase with Store {
       : state = InitialExecutionState(),
         currencies = _wallet.balance.keys.toList(),
         selectedCryptoCurrency = _wallet.currency,
-        outputs = ObservableList<Output>() {
+        outputs = ObservableList<Output>(),
+        fiatFromSettings = _settingsStore.fiatCurrency {
     final priority = _settingsStore.priority[_wallet.type];
     final priorities = priorityForWalletType(_wallet.type);
 
@@ -52,7 +53,7 @@ abstract class SendViewModelBase with Store {
     
     outputs.add(Output(_wallet, _settingsStore, _fiatConversationStore, () => selectedCryptoCurrency));
   }
-
+  
   @observable
   ExecutionState state;
 
@@ -133,11 +134,13 @@ abstract class SendViewModelBase with Store {
 
   Validator get textValidator => TextValidator();
 
+  final FiatCurrency fiatFromSettings;
+
   @observable
   PendingTransaction? pendingTransaction;
 
   @computed
-  String get balance => balanceViewModel.availableBalance ?? '0.0';
+  String get balance => balanceViewModel.availableBalance;
 
   @computed
   bool get isReadyForSend => _wallet.syncStatus is SyncedSyncStatus;
@@ -166,6 +169,9 @@ abstract class SendViewModelBase with Store {
 
   bool get hasCurrecyChanger => walletType == WalletType.haven;
 
+  @computed
+  FiatCurrency get fiatCurrency => _settingsStore.fiatCurrency;
+
   final WalletBase _wallet;
   final SettingsStore _settingsStore;
   final SendTemplateViewModel sendTemplateViewModel;
@@ -208,7 +214,7 @@ abstract class SendViewModelBase with Store {
       state = TransactionCommitting();
       await pendingTransaction!.commit();
 
-      if (pendingTransaction!.id?.isNotEmpty ?? false) {
+      if (pendingTransaction!.id.isNotEmpty) {
         _settingsStore.shouldSaveRecipientAddress
             ? await transactionDescriptionBox.add(TransactionDescription(
                 id: pendingTransaction!.id,
@@ -283,4 +289,12 @@ abstract class SendViewModelBase with Store {
 
   bool _isEqualCurrency(String currency) => 
       currency.toLowerCase() == _wallet.currency.title.toLowerCase();
+
+  @action
+  void onClose() =>
+      _settingsStore.fiatCurrency = fiatFromSettings;
+  
+  @action
+  void setFiatCurrency(FiatCurrency fiat) =>
+      _settingsStore.fiatCurrency = fiat;
 }