Update client.dart

Co-authored-by: AhmedHanafy725 <41957921+AhmedHanafy725@users.noreply.github.com>

Author: AlaaElattar

packages/stellar_client/lib/src/client.dart

@@ -367,7 +367,7 @@ class Client {
     List<BalanceInfo> balance = [];
     try {
       // check that receiver account exists
-      balance = await getBalanceByAccountID(
+      receiverBalance = await getBalanceByAccountID(
           network: _network, accountId: destinationAddress);
     } catch (e) {
       throw Exception('Receiver account does not exist.');