fix ElectrumEstimateFee

Author: mrtnetwork

Diff for: example/lib/global/transfer_from_7_account_to_6_accout_example.dart

@@ -123,6 +123,11 @@ void main() async {
   /// get network fee esmtimate (fee per kilobyte)
   final networkEstimate = await provider.request(ElectrumEstimateFee());
 
+  /// the daemon does not have enough information to make an estimate
+  if (networkEstimate == null) {
+    return;
+  }
+
   /// Convert kilobytes to bytes, multiply by the transaction size, and the result yields the transaction fees.
   final fee =
       BigInt.from(transactionSize) * (networkEstimate ~/ BigInt.from(1000));

Diff for: example/lib/global/transfer_to_8_account_example.dart

@@ -102,6 +102,11 @@ void main() async {
   /// get network fee esmtimate (kb/s)
   final networkEstimate = await provider.request(ElectrumEstimateFee());
 
+  /// the daemon does not have enough information to make an estimate
+  if (networkEstimate == null) {
+    return;
+  }
+
   /// kb to bytes and mul with transaction size and now we have fee
   final fee =
       BigInt.from(estimateSize) * (networkEstimate ~/ BigInt.from(1000));

Diff for: lib/src/provider/electrum_methods/methods/estimate_fee.dart

@@ -3,7 +3,7 @@ import 'package:bitcoin_base/src/utils/btc_utils.dart';
 
 /// Return the estimated transaction fee per kilobyte for a transaction to be confirmed within a certain number of blocks.
 /// https://electrumx-spesmilo.readthedocs.io/en/latest/protocol-methods.html
-class ElectrumEstimateFee extends ElectrumRequest<BigInt, dynamic> {
+class ElectrumEstimateFee extends ElectrumRequest<BigInt?, dynamic> {
   ElectrumEstimateFee({this.numberOfBlock = 2});
 
   /// The number of blocks to target for confirmation.
@@ -20,7 +20,9 @@ class ElectrumEstimateFee extends ElectrumRequest<BigInt, dynamic> {
 
   /// The estimated transaction fee in Bigint(satoshi)
   @override
-  BigInt onResonse(result) {
-    return BtcUtils.toSatoshi(result.toString()).abs();
+  BigInt? onResonse(result) {
+    final fee = BtcUtils.toSatoshi(result.toString());
+    if (fee.isNegative) return null;
+    return fee;
   }
 }