diff --git a/src/core/utils_linux.cpp b/src/core/utils_linux.cpp
index d8d66c7..625b1c8 100644
--- a/src/core/utils_linux.cpp
+++ b/src/core/utils_linux.cpp
@@ -531,11 +531,6 @@ static inline void themeChangeNotificationCallback()
 
 void Utils::registerThemeChangeNotification()
 {
-    static bool reg = false;
-    if (reg) {
-        return;
-    }
-    reg = true;
     GtkSettings * const settings = gtk_settings_get_default();
     Q_ASSERT(settings);
     if (!settings) {
diff --git a/src/core/utils_win.cpp b/src/core/utils_win.cpp
index 0e9decf..5e7c033 100644
--- a/src/core/utils_win.cpp
+++ b/src/core/utils_win.cpp
@@ -1829,6 +1829,12 @@ void Utils::disableOriginalTitleBarFunctionalities(const WId windowId, const boo
 void Utils::setQtDarkModeAwareEnabled(const bool enable, const bool pureQuick)
 {
 #if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
+    // We'll call QPA functions, so we have to ensure that the QGuiApplication
+    // instance has already been created and initialized, because the platform
+    // integration infrastructure is created and maintained by QGuiApplication.
+    if (!qGuiApp) {
+        return;
+    }
     using App = QNativeInterface::Private::QWindowsApplication;
     if (const auto app = qApp->nativeInterface<App>()) {
         app->setDarkModeHandling([enable, pureQuick]() -> App::DarkModeHandling {