confirm_tx dialog: provide feedback to the user when tx is not ready

electrum/gui/qt/confirm_tx_dialog.py

@@ -532,7 +532,7 @@ class TxEditor(WindowModalDialog):
             if self.not_enough_funds:
                 self.io_widget.update(None)
             self.set_feerounding_visibility(False)
-            self.messages = []
+            self.messages = [_('Preparing transaction...')]
         else:
             self.messages = self.get_messages()
             self.update_fee_fields()
@@ -619,7 +619,7 @@ class ConfirmTxDialog(TxEditor):
             title=_("New Transaction"), # todo: adapt title for channel funding tx, swaps
             allow_preview=allow_preview)
 
-        self.update()
+        self.trigger_update()
 
     def _update_amount_label(self):
         tx = self.tx