Frontend.XF: fix recently introduced crash

Somehow mainLayout was still queried at the click-handler
when it had already been at the SendPage class level. This
should have been done in [1] ([2]).

[1] 2d3b90c
[2] 262348b

src/GWallet.Frontend.XF/SendPage.xaml.fs

@@ -754,7 +754,6 @@ type SendPage(account: IAccount, receivePage: Page, newReceivePageFunc: unit->Pa
         } |> FrontendHelpers.DoubleCheckCompletionAsync false
 
     member self.OnSendOrSignButtonClicked(_sender: Object, _args: EventArgs): unit =
-        let mainLayout = base.FindByName<StackLayout>("mainLayout")
         let amountToSend = mainLayout.FindByName<Entry>("amountToSend")
         let destinationAddress = destinationAddressEntry.Text