gui: disable top bar menu actions during shutdown

Opening the top bar menu when the app is being destroyed
freezes the GUI shutdown process for no reason. No menu
action can be executed.

Note:
This behavior is consistent with how the tray icon menu
is cleared too.

src/qt/bitcoingui.cpp

@@ -650,6 +650,7 @@ void BitcoinGUI::setClientModel(ClientModel *_clientModel, interfaces::BlockAndH
 
         m_mask_values_action->setChecked(_clientModel->getOptionsModel()->getOption(OptionsModel::OptionID::MaskValues).toBool());
     } else {
+        // Shutdown requested, disable menus
         if (trayIconMenu)
         {
             // Disable context menu on tray icon
@@ -664,6 +665,8 @@ void BitcoinGUI::setClientModel(ClientModel *_clientModel, interfaces::BlockAndH
         }
 #endif // ENABLE_WALLET
         unitDisplayControl->setOptionsModel(nullptr);
+        // Disable top bar menu actions
+        appMenuBar->clear();
     }
 }