void OnFileExportBlockchain(object sender, RoutedEventArgs e)
{
var wf = SelectedWalletNotNull();
var d = new SaveFileDialog();
d.FileName = "bootstrap.dat";
d.Filter = "Bitcoin Bootstrap.dat format|bootstrap.dat";
if (Dialog.ShowDialog(d, this))
wf.Wallet.ExportToBootstrapDat(d.FileName);
}