fix: stop refetching the desktop fee rates on every rebuild

### lib/pages_desktop_specific/my_stack_view/wallet_view/sub_widgets/desktop_send_fee_form.dart
@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../../../../models/paymint/fee_object_model.dart';
 import '../../../../pages/send_view/sub_widgets/transaction_fee_selection_sheet.dart';
 import '../../../../providers/providers.dart';
 import '../../../../providers/ui/fee_rate_type_state_provider.dart';
@@ -56,6 +57,7 @@ class _DesktopSendFeeFormState extends ConsumerState<DesktopSendFeeForm> {
   ];
 
   late final CryptoCurrency cryptoCurrency;
+  Future<FeeObject>? _fees;
 
   bool get isEth => cryptoCurrency is Ethereum;
 
@@ -174,12 +176,15 @@ class _DesktopSendFeeFormState extends ConsumerState<DesktopSendFeeForm> {
             padding: const EdgeInsets.all(10),
             child: (feeSelectionResult?.$2 == null)
                 ? FutureBuilder(
-                    future: ref.watch(
-                      pWallets.select(
-                        (value) => value.getWallet(widget.walletId).fees,
-                      ),
-                    ),
+                    future: _fees ??= ref
+                        .read(pWallets)
+                        .getWallet(widget.walletId)
+                        .fees,
                     builder: (context, snapshot) {
+                      if (snapshot.connectionState == ConnectionState.done &&
+                          snapshot.hasError) {
+                        _fees = null;
+                      }
                       if (snapshot.connectionState == ConnectionState.done &&
                           snapshot.hasData) {
                         return DesktopFeeItem(

### test/pages_desktop_specific/wallet/desktop_send_fee_form_test.dart
@@ -0,0 +1,144 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:stackwallet/models/isar/models/blockchain_data/address.dart';
+import 'package:stackwallet/models/isar/stack_theme.dart';
+import 'package:stackwallet/models/paymint/fee_object_model.dart';
+import 'package:stackwallet/pages_desktop_specific/my_stack_view/wallet_view/sub_widgets/desktop_send_fee_form.dart';
+import 'package:stackwallet/providers/global/wallets_provider.dart';
+import 'package:stackwallet/providers/wallet/desktop_fee_providers.dart';
+import 'package:stackwallet/themes/stack_colors.dart';
+import 'package:stackwallet/utilities/amount/amount.dart';
+import 'package:stackwallet/utilities/amount/amount_formatter.dart';
+import 'package:stackwallet/utilities/amount/amount_unit.dart';
+import 'package:stackwallet/wallets/crypto_currency/crypto_currency.dart';
+import 'package:stackwallet/wallets/isar/models/wallet_info.dart';
+import 'package:stackwallet/wallets/isar/providers/wallet_info_provider.dart';
+import 'package:stackwallet/wallets/wallet/wallet.dart';
+import 'package:stackwallet/widgets/desktop/desktop_fee_dialog.dart';
+
+import '../../sample_data/theme_json.dart';
+import '../../wallets/support/xelis_test_fakes.dart';
+
+const _walletId = "wallet id";
+final _coin = Bitcoin(CryptoCurrencyNetwork.main);
+final _info = WalletInfo(
+  walletId: _walletId,
+  name: "Bitcoin wallet",
+  mainAddressType: AddressType.p2wpkh,
+  coinName: _coin.identifier,
+);
+
+/// Bitcoin wallet counting its fee rate fetches, the first [failingFetches]
+/// of which fail.
+class _FeeWallet implements Wallet<Bitcoin> {
+  _FeeWallet({this.failingFetches = 0});
+
+  final int failingFetches;
+  var feeFetches = 0;
+
+  @override
+  WalletInfo get info => _info;
+
+  @override
+  Future<FeeObject> get fees async {
+    feeFetches++;
+    if (feeFetches <= failingFetches) throw Exception("node unreachable");
+    return FeeObject(
+      numberOfBlocksFast: 1,
+      numberOfBlocksAverage: 5,
+      numberOfBlocksSlow: 20,
+      fast: BigInt.from(20000),
+      medium: BigInt.from(10000),
+      slow: BigInt.from(5000),
+    );
+  }
+
+  @override
+  Future<Amount> estimateFeeFor(Amount amount, BigInt feeRate) async =>
+      Amount(rawValue: BigInt.from(1410), fractionDigits: 8);
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+/// Pump the fee form for [wallet] and return its parent's setState.
+Future<StateSetter> _pumpForm(WidgetTester tester, _FeeWallet wallet) async {
+  final theme = StackTheme.fromJson(json: lightThemeJsonMap);
+  late StateSetter rebuild;
+  await tester.pumpWidget(
+    ProviderScope(
+      overrides: [
+        pWallets.overrideWithValue(ConfirmationWallets(wallet)),
+        pWalletCoin(_walletId).overrideWithValue(_coin),
+        pAmountFormatter(_coin).overrideWithValue(
+          AmountFormatter(
+            unit: AmountUnit.normal,
+            locale: "en_US",
+            coin: _coin,
+            maxDecimals: 8,
+          ),
+        ),
+      ],
+      child: MaterialApp(
+        theme: ThemeData(extensions: [StackColors.fromStackColorTheme(theme)]),
+        home: Material(
+          child: StatefulBuilder(
+            builder: (context, setState) {
+              rebuild = setState;
+              return DesktopSendFeeForm(
+                walletId: _walletId,
+                isToken: false,
+                onCustomFeeSliderChanged: (_) {},
+                onCustomFeeOptionChanged: () {},
+              );
+            },
+          ),
+        ),
+      ),
+    ),
+  );
+  await tester.pump();
+  return rebuild;
+}
+
+void main() {
+  testWidgets("a rebuild keeps the fee rates and the amount", (tester) async {
+    final wallet = _FeeWallet();
+    final rebuild = await _pumpForm(tester, wallet);
+    final container = ProviderScope.containerOf(
+      tester.element(find.byType(DesktopSendFeeForm)),
+    );
+    final amount = Amount(rawValue: BigInt.from(22000), fractionDigits: 8);
+    container.read(sendAmountProvider.notifier).state = amount;
+    await tester.pump();
+
+    rebuild(() {});
+    await tester.pump();
+    await tester.pump();
+
+    expect(wallet.feeFetches, 1);
+    expect(container.read(sendAmountProvider), amount);
+  });
+
+  testWidgets("a rebuild fetches the fee rates again after a failure", (
+    tester,
+  ) async {
+    final wallet = _FeeWallet(failingFetches: 1);
+    final rebuild = await _pumpForm(tester, wallet);
+    expect(find.byType(DesktopFeeItem), findsNothing);
+
+    rebuild(() {});
+    await tester.pump();
+    await tester.pump();
+
+    expect(wallet.feeFetches, 2);
+    expect(find.byType(DesktopFeeItem), findsOneWidget);
+
+    rebuild(() {});
+    await tester.pump();
+
+    expect(wallet.feeFetches, 2);
+    expect(find.byType(DesktopFeeItem), findsOneWidget);
+  });
+}