show warning about offline mode instead of returning silently

gui/qt/main_window.py

@@ -2696,6 +2696,7 @@ class ElectrumWindow(QMainWindow):
 
     def run_network_dialog(self):
         if not self.network:
+            QMessageBox.warning(self, _('Offline'), _('You are using Electrum in offline mode.\nRestart Electrum if you want to get connected.'), _('OK'))
             return
         NetworkDialog(self.wallet.network, self.config, self).do_exec()