implement spinner control.
MODIFY
WalletWasabi.Gui/MainWindow.xaml
WalletWasabi.Gui/MainWindow.xaml
<shell:ShellView />
</DockPanel>
<wasabi:ModalDialog DataContext="{Binding ModalDialog}" />
+ <wasabi:Spinner HorizontalAlignment="Center" VerticalAlignment="Center" />
</Grid>
</cont:MetroWindow>

implement spinner control.