qt PreviewTxDialog: check for "not enough funds" also when shortcutting

see https://github.com/spesmilo/electrum/issues/6306#issuecomment-652424363

scenario: user saves invoice for more money than they have, has advanced_preview
in config enabled, tries to pay it, half-empty tx preview dialog opens

electrum/gui/qt/main_window.py

@@ -1613,10 +1613,6 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         if output_values.count('!') > 1:
             self.show_error(_("More than one output set to spend max"))
             return
-        if self.config.get('advanced_preview'):
-            self.preview_tx_dialog(make_tx=make_tx,
-                                   external_keypairs=external_keypairs)
-            return
 
         output_value = '!' if '!' in output_values else sum(output_values)
         d = ConfirmTxDialog(window=self, make_tx=make_tx, output_value=output_value, is_sweep=is_sweep)
@@ -1626,6 +1622,13 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             if not d.have_enough_funds_assuming_zero_fees():
                 self.show_message(_('Not Enough Funds'))
                 return
+
+        # shortcut to advanced preview (after "enough funds" check!)
+        if self.config.get('advanced_preview'):
+            self.preview_tx_dialog(make_tx=make_tx,
+                                   external_keypairs=external_keypairs)
+            return
+
         cancelled, is_send, password, tx = d.run()
         if cancelled:
             return