Qt: Fix performance counters showing up without VM

src/duckstation-qt/qthost.cpp

@@ -353,6 +353,8 @@ void EmuThread::checkForSettingsChanges(const Settings& old_settings)
   if (g_main_window)
   {
     QMetaObject::invokeMethod(g_main_window, &MainWindow::checkForSettingChanges, Qt::QueuedConnection);
+
+    if (System::IsValid())
       updatePerformanceCounters();
   }