fix CalculateNetworkFee

Author: Jim8y

src/RpcServer/RpcServer.Wallet.cs

@@ -128,9 +128,11 @@ protected virtual JToken CalculateNetworkFee(JArray _params)
             var tx = Convert.FromBase64String(_params[0].AsString());
 
             JObject account = new();
-            var networkfee = Wallets.Helper.CalculateNetworkFee(
+            wallet ??= new DummyWallet(system.Settings);
+            var networkfee =
+                Wallets.Helper.CalculateNetworkFee(
                 tx.AsSerializable<Transaction>(), system.StoreView, system.Settings,
-                wallet is not null ? a => wallet.GetAccount(a).Contract.Script : null);
+                a => wallet.GetAccount(a).Contract.Script);
             account["networkfee"] = networkfee.ToString();
             return account;
         }